Converting to a marine aquarium from freshwater

A comprehensive guide for anyone with a freshwater aquarium interested in converting to a marine aquarium

Thinking about converting to a marine aquarium?

When I was a kid, I got hooked into the aquarium hobby by starting out with a freshwater aquarium. As I have gotten to know many of the stories shared with me by subscribers of the saltwater aquarium blog newsletter, I have come to realize that many of us have joined this hobby after first starting out and having success with a freshwater tank.

The goal of this article is to highlight some of the similarities and differences between converting to a reef tank and a freshwater aquarium.

If you enjoy creating a freshwater habitat and watching the animals and plants in the aquarium thrive, then you’ll also enjoy a saltwater aquarium. Since you already have experience in keeping freshwater fish and plants alive, you’ll know that it’s not just about collecting animals and putting them in the tank, it’s a lot more than that–so let’s take a deeper dive into the similarities and differences between keeping a saltwater aquarium and a freshwater aquarium so you can consider converting to a marine aquarium with confidence.

converting to a marine aquarium from freshwater
This freshwater tank is beautiful, but do you ever dream of having a saltwater aquarium?

At the risk of stating the obvious, the first and most important difference between keeping a saltwater aquarium and a freshwater aquarium is that you fill your saltwater aquarium with saltwater.

You make the saltwater by mixing a high-quality reef salt with purified freshwater. If you are just starting out and want to know how to mix your own saltwater, check out this tutorial here.

So what are the similarities and differences between keeping a saltwater aquarium and a freshwater aquarium? Let’s take a closer look at a few important aspects of the hobby. 

Water quality

Similarities to a freshwater aquarium

Whether you are keeping a marine aquarium or a freshwater aquarium, the name of the game is water quality. You could have plans to keep the most amazing animals in the world, but if the quality of the water in your tank is inadequate, you will struggle to keep saltwater fish and corals.

Differences to expect when converting to a marine aquarium

One of the biggest differences between the average freshwater aquarium and the average reef tank, in terms of water quality, is that many saltwater fish and corals are more sensitive to changes in water parameters than the average freshwater fish–so you need to maintain tight control over the quality of the water and make sure the water parameters are strong in all of the 9 most important water parameters with a saltwater aquarium.

Don’t take this the wrong way–I’m not advocating for poor water quality in the freshwater aquarium side of the hobby–pristine water quality is a prerequisite for responsible keeping of any aquatic organisms, but I think freshwater hobbyists, at least the hobbyist I was and a few that I knew, were a little more lax in their husbandry.

In the saltwater aquarium hobby, you need to keep on top of the maintenance and the water parameters or you will suffer expensive losses.

For more information about the most important water parameters for a saltwater aquarium, check out this article here.

Important to test water parameters


The only way you’re going to know if your water quality is pristine is if you test the water routinely, using high-quality test kits or strips. This need to test is the same, whether you have a freshwater aquarium or a saltwater aquarium.

Ammonia, nitrites, and nitrates are still the key water parameters for waste management and to ensure your biological filter is working properly. Measuring temperature and pH are still key to ensuring your vital parameters don’t fluctuate too much.


When converting to a marine aquarium there are some additional water parameters to keep track of. In addition to the water parameters listed above, you also want to be sure you test for Hardness, Calcium, Salinity, and Phosphates. You will want to buy new kits for your saltwater aquarium and keep good records of your testing schedule and your water parameters in a Reef Journal.


Partial water changes


The best thing you can do for either your freshwater aquarium or saltwater aquarium is to perform a partial water change. Dilution is the solution to pollution and partial water changes will work wonders on revitalizing a freshwater or marine aquarium.


The big difference when converting to a marine aquarium is that the animals in your aquarium (corals, other invertebrates, and fish) are generally much less tolerant of high nitrate levels than the fish in freshwater. The principle is still the same–you should perform water changes to keep nitrates close to zero, but in practice, I know this is something that can slip in a freshwater aquarium maintenance schedule because the consequences don’t appear as quickly and clearly.

With a freshwater tank, you drain freshwater and replace it with fresh water. Perhaps you dechlorinated the tap water (always a good practice) but for the most part, you just replaced the water and didn’t think that much about it.

When replacing saltwater from a partial water change, you want to be mindful of the salinity and pH of the water in your aquarium and the water you are replacing it with, to avoid shocking the animals in your tank.

Since making your replacement water involves mixing and dissolving the reef salt mix, you want to be sure the water parameters are the same before replacing the water.

Cleaning the glass


Did you have a plecostomus, snails, or a Chinese algae eater to keep your aquarium glass clean and free from algae? Many saltwater aquarium enthusiasts keep snails–and certain snail species will scrape algae off the glass.


The snails you have in your saltwater tank won’t keep the glass clean. In my opinion, the glass for my saltwater aquarium gets covered with algae much more than my freshwater aquarium ever did.

Coralline algae on glass

So you will need to clean your glass more often. There is also a cool, beneficial type of calcareous algae that grows in a saltwater aquarium that will probably require you to literally scrape it off the glass in certain places.

Biological filtration is still the key when converting to a marine aquarium


Biological filtration is the key to keeping nutrient levels in check, whether you have a freshwater or a marine aquarium. Beneficial bacteria will colonize your freshwater aquarium or your saltwater aquarium and convert that waste from toxic ammonia into nitrites and nitrates.


What’s different about filtration in a marine aquarium is that you probably won’t use a filter, per se. When I was younger and lower tech, I had a simple in-tank filter that had filter floss and activated carbon, powered by a small air pump. Eventually, I upgraded that to a hang-on-the-back style power filter.

Either model would work in a marine aquarium, in fact, I use a sponge filter in my quarantine/hospital tank, but if you have dreams of a conventional saltwater aquarium with corals and fish, you will probably ditch the power filter, or your elitist saltwater aquarium friends will make fun of you.

That’s because most saltwater aquaria that I’ve seen don’t actually use a power filter.

Instead, they keep substrates like live rock and live sand in the aquarium to act as a substrate for the beneficial bacteria serving as the backbone for the biological filter and powerheads (powerful in-tank water pumps) to push the water around for oxygenation and to create water flow.



The basic husbandry needs of fish are quite similar, whether you have a freshwater aquarium or are converting to a marine aquarium. They need clean water and to be fed regularly with high-quality food.

butterfly fish in a marine aquarium

Small amounts of food fed more frequently are generally better than large infrequent meals in either case.


The biggest differences when converting to a marine aquarium are:

General hardiness/tolerance for aquarium conditions

This is a generalization of a generalization, but I would say that there is a difference in overall hardiness and tolerance for aquarium conditions between the average freshwater fish and the average saltwater fish.

Unfortunately, many of the saltwater fish species we keep today are caught from a reef and shipped to the local fish store or your home. By comparison, the average freshwater aquarium fish was most likely aquacultured and was therefore born and raised in captivity.

Of course, there are exceptions to this observation–for example, the common clownfish is aquacultured and also is generally tolerant of life in the aquarium–and if you compare this with keeping the Discus, a South American Cichlid, you would likely see that this is one of the more challenging freshwater aquarium fish.

But rather than take your success in the freshwater hobby for granted, I’d rather you consider that, to successfully transition from a freshwater aquarium to a saltwater aquarium, you would be best served to fine-tune your husbandry efforts a bit.


Aggression between species does happen in the freshwater aquarium space–particularly among notoriously aggressive fish, like cichlids, but there are large numbers of community fish available in the freshwater aquarium hobby that can be kept in schools of similar fish and mixed in a community tank.

Territoriality and aggression between fish of the same species and even across species is a bit more common in the saltwater aquarium space. There are very few schooling species that can be kept in the average saltwater aquarium. Hobbyists often try to keep schools of green Chromis, blue streak cardinalfish or even Banggai cardinalfish, only to find the school dwindling in numbers over the coming days and weeks.

While it is generally acceptable to mix tangs, angelfish, basslets, and cardinalfish with each other, it is typically best just to keep one specimen from a given group to avoid aggression between fish of the same species.

Specialized dietary needs

Many of the desirable saltwater aquarium species have specialized dietary needs which can make them challenging, expensive and high maintenance to feed. Many fish will refuse to eat commercially prepared foods like flakes and pellets. Some will only take live foods, others require sponges, algae or polyps in their diets.

moorish idol has special needs that are best kept by advanced hobbyists only
Moorish idol is an insanely popular fish with an insanely poor track record

Don’t get scared away by this–many of the most common fish are easy to feed. The fish in my tank are all accustomed to eating flakes, pellets, frozen and live foods–but it might take a while to get them used to it.

Diseases and parasites


Life in an aquarium seems to exaggerate the risk of spreading disease and parasites, whether you’re looking at fish for the saltwater aquarium or freshwater aquarium. Purchasing healthy fish, quarantining those fish and treating any sick fish in isolation prior to adding them the home aquarium is a best practice that should be implemented in the freshwater aquarium or saltwater aquarium hobby.


The main difference when converting to a marine aquarium is that you can’t easily treat a parasitic infection in a saltwater aquarium. Most (all?) of the parasites that attack your saltwater fish and corals are invertebrates and most of the medications that would treat (kill) parasites also kill the other invertebrates in your tank–so the cure for saltwater ich would also kill the shrimp, crabs, clams or beneficial copepods in your tank.

That’s a pretty big difference. It’s also fairly difficult to capture fish in a saltwater tank, making the removal of an infected fish quite difficult.

The need to set up a quarantine tank and using an effective quarantine process is a critical and important difference between a saltwater aquarium and a freshwater aquarium. The need is there for both sides of the hobby–and quarantine is a best practice for either–but because you can typically treat the fish in your display tank without a whole lot of hassle, the risks of not quarantining are much lower.



At the risk of stating the obvious here–there are no corals in the freshwater aquarium hobby.

Duh. I know you know that. But what I can say is that keeping corals, in some ways, is similar to keeping live plants in the freshwater aquarium. You have to make sure you are not keeping any fish that will eat the plants (freshwater) or corals (saltwater).

You will need high-quality lights to support photosynthesis. Just like plants, many of the corals (particularly the soft corals) will absorb and remove nutrients/waste from the water, essentially purifying the water some.

A great soft coral species to start out with is something called green star polyps. Learn more about GSP here.


The biggest differences you will experience when converting to a marine aquarium are:

Corals are animals, not plants

Corals are animals, not plants and animals need to eat. Therefore most corals also need to be fed. The symbiotic relationship they have with photosynthetic algae, called zooxanthellae are just part of the nutritional equation–so you should plan on feeding your corals.  


Many corals are more sensitive to being shocked by major changes in water conditions and will not tolerate less than pristine water conditions.

Nutrients in the water

Plants benefit from having fertilizer-type nutrients in the water–nitrogen, and phosphorus compounds as well as carbon dioxide. Corals, on the other hand, generally want low nitrates or phosphates and high levels of also need calcium in the water to help build their bony skeletons. Calcium is often added to saltwater aquarium water as a supplement. The basic supplement used is called kalkwasser, or calcium hydroxide.



Equipment you will keep

When converting to a marine aquarium from freshwater, you can put some of the equipment from your freshwater aquarium right to work in your saltwater aquarium, as long as you have a standard glass or acrylic aquarium. Some tanks have mirrored back walls, which are not suitable for saltwater, but otherwise, most tanks and stands will be fine.

You will also get good use out of your thermometer and heater.


Equipment you will need to upgrade


When converting to a marine aquarium, unless you had a planted tank, with some killer LED, metal halide or t5 lights (if you don’t know what these are…that’s probably a good sign you will want to upgrade your lights), you will most likely need to upgrade your lights.

When I had a freshwater tank, I typically just used a standard hood and whatever light came with the standard hood. Unless you are setting up a very basic, fish-only saltwater aquarium, you will need to upgrade your lights.

Aquasun LED Light

If you have access to inexpensive or used high output or very high output lights, most models work well for most saltwater aquarium situations. If you need to purchase a new light to convert from a freshwater aquarium to a saltwater aquarium, I strongly recommend you explore the LED options available today.

It may cost you a bit more upfront (shop around to find the right features for the price you’re willing to pay), but they generally pay for themselves in cost savings in a few years, compared with the energy-guzzling and bulb replacement costs of some of the other options.

Equipment you will not use


If you have a hang-on-the-back power filter, under gravel filter, or in-tank foam filter, you can use it on your quarantine tank, but you probably won’t use it on the freshwater aquarium converted to saltwater aquarium use.

There is no reason you couldn’t use it, but most saltwater aquarium hobbyists simply don’t. Instead, you will likely have live rock and live sand, which will serve as the substrate the beneficial bacteria in your biological filter will grow on.

New equipment to buy when converting to a marine aquarium

Test kits

To test your saltwater aquarium water, you will want ammonia, nitrite, nitrate, pH, hardness, calcium and phosphate tests. Your old pH test won’t work. The pH of freshwater is ~7.0, the pH of saltwater should be about 8.2 and the old test won’t work.


Powerheads are water pumps that you put inside your aquarium. You might actually have these already in your freshwater aquarium–if so–they are totally ok to reuse. If you don’t already have them, you will want to buy one or more, depending on the size of your tank.

Water movement is critically important if you plan to keep corals because most corals won’t be able to move, they depend on the water to bring nutrients to them and take away waste. Strong water movement also helps keep problem algae growth under control.

Protein Skimmer (optional)

A protein skimmer is a piece of equipment that removes organic waste from the water. This specialized piece of equipment doesn’t work well in freshwater (which is why you may have never heard about it before) but works very well with salt water. It’s completely optional, but adding a protein skimmer will help you keep your tank cleaner and will provide an additional margin for error that you might appreciate.

Check out some protein skimmer product reviews here

Sump (optional)

When converting to a marine aquarium, many hobbyists set up a sump, which is a smaller ‘tank’, under your tank, where you keep equipment, like your heater, thermometer, protein skimmer, and refugium, out of sight. The sump is also partially filled with saltwater all of the time, which effectively increases the ‘size’ (volume) of your tank–which should allow you to keep more animals in your tank.

This is not a requirement. When I first converted to a marine aquarium from a freshwater aquarium, I did it without a sump, but a few years later, I upgraded my saltwater aquarium to a reef-ready (drilled) tank with an overflow and installed a sump.

Refugium (optional)

A refugium is a small ‘tank within a tank’. It’s like the equivalent of having a wildlife preserve within your tank. A place where you allow macroalgae and beneficial invertebrates and bacteria to grow without the pressures from the livestock in the display area of the aquarium. A refugium is a natural way to remove impurities from the water and to boost the natural biodiversity in your tank.

Many people put the refugium in their sump (which is why I listed it here as a sub-section of the sump area), although there are also hang-on-the-back refugium models you could purchase if that doesn’t work for you.

A refugium isn’t exactly ‘equipment’, per se, but it typically involves customization of the equipment you already have and it may require a dedicated light.



Just like for your freshwater aquarium, you will need to buy food, test kits and maintenance equipment to keep the glass clean and to perform partial water changes.


A marine aquarium is significantly more expensive than a freshwater aquarium. Here are a few areas where the incremental expenses kick in:



The water from your tap is nearly free, or at the least, very low cost


The water you will use with a saltwater aquarium may be purified first by RO/DI (an expensive piece of equipment) and you will mix it with a reef salt mix, which will cost you an additional $0.25-$0.40 per gallon



The average freshwater fish costs a few dollars. You have to really try hard to find fish in the $20-$50 range.


When converting to a marine aquarium, there are very few fish you can purchase for less than $10. There are several popular fish available for about $20 or less, and there are a lot of fish in the $40 and up range. Corals cost $5-$20 for a very small frag, $40-50 for an average small colony and $75+ for larger or rarer colonies.



For your freshwater aquarium, you likely bought a few bags of aquarium gravel (pebbles), some driftwood or rocks and a decoration or two.


When converting to a marine aquarium, you will likely replace your pebble gravel with crushed coral or sand (some people even buy very expensive ‘live sand’ products). You will also likely want a relatively large quantity of live rock, which will cost you a few dollars per pound.

For more information, check out this popular post that addresses the question: how much live rock do you need?



Just about any lights will be fine unless you want a planted tank with photosynthetic plants.


When converting to a marine aquarium, your lights will probably cost a few hundred dollars. You can learn a bit more about your lighting options on the equipment page



The only electricity strain caused by a freshwater tank is from the heater, lights (typically low-wattage fluorescent light) and maybe the pump for the filter.


The electricity costs to run a saltwater aquarium can get rather high. A typical aquarium light will consume one hundred or more kilowatts per hour, you will generally run additional pumps and powerheads, which will keep that electric meter spinning.

What to do with your freshwater livestock

One of the biggest dilemmas in your decision to convert from a freshwater aquarium to a saltwater aquarium (hopefully) is what to do with the remaining freshwater fish in your tank. Here are a few options to consider:

Wait for your fish to expire naturally

One of the easiest things you can do is just wait for your fish to live a full life in your tank and convert your freshwater aquarium to a saltwater aquarium after the last animal dies naturally in your tank.

This, of course, is also the slowest way to convert your tank. The good news is that you’ll have plenty of time to research, learn and plan out your marine aquarium conversion.

Give the fish away to a friend

One option, to speed things along, is to see if you can give your remaining freshwater fish away to a friend with a suitably sized freshwater aquarium so you can continue converting to a marine aquarium. If you don’t have a friend with a tank (or who is willing to take on your fish), a similar approach could be achieved by finding a local aquarium group online and see if you can sell or give the fish away.

See if your local fish store will take them back

Sometimes, you can trade your fish back in for credit or simply give them back to your local fish store, who will then send those fish to a good home.


As a last resort, if you absolutely must convert your freshwater aquarium over to a marine aquarium quickly, you could consider euthanizing the fish, which is a fancy term for finding a humane way to expire the fish.

I don’t mean to be too judgy here, but this option kind of stinks.

I’m not that big of a fan here–but I think the reason I want to include this here is in the hope of encouraging responsible and

humane disposal.

Never, ever, ‘release’ the fish into a local waterway and please don’t flush a living fish down the drain. Try to get the fish to another suitable aquarium, but if you have no other options, consider euthanizing.


I hope this article helped put into perspective the similarities and differences between operating a freshwater and saltwater aquarium and gave you confidence when converting to a marine saltwater aquarium.

marine reef tank also known as a saltwater aquarium
Image by David Stanley

Converting to a marine aquarium: continue your research

The journey has really just begun. There is a lot to learn and a lot you should learn before finally taking the plunge. If you’re thinking about getting started with a marine aquarium here are a few pages you should check out first:

How to set up a saltwater aquarium

Best aquarium resources

Equipment page

I also strongly recommend you sign up for the Saltwater Aquarium Blog Newsletter. That’s where I share tips, tricks, and strategies to help you have a more successful saltwater aquarium.


Written by Albert B. Ulrich III–author of The Reef Aquarium Series of books:  The New Saltwater Aquarium Guide, How to Frag Corals, 107 Tips for the Marine Reef Aquarium and The Reef Journal





6 responses to “Converting to a marine aquarium from freshwater”

  1. Alex Miller

    I was actually thinking about going back to fresh for a small tank! My 9 gallon quarantine tank (aka glass box of death) won’t be used for that purpose again, and it is a lovely aquarium. (Yes, my last purchase of fishes went straight in the main tank without incident. As opposed to typical quarantine fail. But that’s another article.)
    Might try a simple fresh water instead of another reef. My home office could use some life.
    Also have an empty 40 gallon in the basement since my upgrade. My husband says get a lizard. 🙂 also a predator tank would be cool.

    … Also we are moving and I am nervous about relocating my tank. I’ve moved a lot of tanks. But this is the first time I’ve had a proper reef with fragile corals. I have several pieces of a branching coral that snaps off if you look at it funny.
    I may have to call in some pros this time.

    1. Alex, thanks for the comment. A fresh, small tank would be fun. Moving a tank isn’t that bad…as long as you plan…how far is the move?

  2. Alex Miller

    I would also add that I found saltwater to be easier to stabilize than fresh in the past.
    Partly the result of not being as educated. But also because of concepts like “tap water is fine.”
    Using tap water, even filtered and conditioned, was the culprit to some frustrating fish kills. Even in a cycled tank, tetras did not do well at all. One of the key fish I wanted. Turns out, the otherwise high quality tap water in that city is very hard. Had a pH of around 8!!! Tetras don’t like that.
    I had much better luck using it in FOWLR saltwater than fresh. Had to haul RO water for success with fresh. Or use the expensive DI cartridges which aren’t even sold anymore.

    1. Alex, thanks for the comment. sorry to hear about your challenges with fish and for sharing the results of switching to RO. I think the ‘laid back’ nature of the freshwater hobby (sometimes) can certainly lead to some unintended challenges.

  3. Angelann

    I have both. My freshwater is 25 gallons. My saltwater is 46 gallons. I just wish both were 46 gallons (lol)!

    1. Angelann, thanks for the comment. There’s always time to go bigger 🙂

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