Aquarium stocking guide

Aquarium Stocking Guide: How Many Fish Per Tank?

When you set up your first saltwater aquarium, the temptation is to pack in as many of your favorite species as possible. But if you refer to the average aquarium stocking guide, you know that practice won’t work. Overcrowded tanks lead to complications with water quality. You can also end up with behavior problems as certain species feel confined. So how DO you decide how many fish you can keep in an aquarium? Not to worry, this handy stocking guide will walk you through all of the ins and outs so you will know exactly how many fish per gallon.

Table of Contents: How many fish per gallon: aquarium stocking guide

The question of how many fish you can get away with in an aquarium is one of the oldest (and hardest) to answer questions in the hobby. For years, reef enthusiasts have sought after the Holy Grail to apply to any tank size. You’ll even find aquarium stocking guides that promise “one-stop shopping” on how to figure out the space you have available. But calculating the available room in a tank isn’t as easy as it looks. And if you want your little ecosystem to remain healthy? You need to consider quite a few variables, as you can see in the links below.

Aquarium stocking guides consider that most fish store fish are juveniles

The “Inches of Fish” Per Gallon Rule

When I started in this hobby, the most common advice passed around was a variation based on the number of inches of fish per gallon/liter of aquarium water. From what I recall, it fell anywhere between 0.5-1 inch (1.2-2cm) of fish per gallon (4L). It sounds reasonable and seems to provide a rough estimate of the needs of the fish you’re adding to the tank. And if you’re flipping through your favorite fish guide and looking at anticipated adult sizes, it allows you to decide whether or not your aquarium can hold the species.

However, there are significant problems with this “rule.” For instance:

  • The way the fish utilizes their size
  • Temperament and potential aggressive tendencies of certain species
  • The daily habits of your fish

And that’s why you won’t see modern aquarium stocking guides refer to it very often. While it works fine as a starting point, you need to take other factors into consideration. If you don’t, you can end up with a tank that struggles to survive. So before you break out your calculator, look at ALL of these points before filling your tank with fish, invertebrates, and plant life.

Aquarium Stocking Guide: How Many Gallons Matters

Yes, the eventual size of your fish matters, but sometimes hobbyists make impulse purchases that land them in hot water. (No, not literally) And it isn’t always their fault. When you scope out fish at your local fish store, you’ll see juvenile or young adult fish. They’re small and perfect for the tank at home. But they’re going to grow and eclipse the size of the aquarium. So how are you supposed to estimate how much fish mass you’re going to have if the number will end up different four, five, or six months down the road?

Aquarium stock guides go beyond the inches of fish rule
Image by Scot Akerman

That’s where the research part of aquarium stocking guides comes in. You need the ability to accept a “barren” tank for a while as your new purchases settle in and grow. And that means resisting the temptation to fill the gaps. If people ask, “When are you going to add more fish?” Simply explain your tank’s at maximum capacity already. Those fish are still growing. Aquarium stocking guides work on ADULT sizing, and you need to keep that in the back of your mind.

Otherwise, you’re going to need a second (or third, or fourth) tank to move your fish to as they expand.

But size problems don’t stop there. Not every fish utilizes their size the same way. We’ll touch on the activity part of the aquarium stock guide in a minute, but it’s something you need to consider when you look at your species. Burrowing fish, such as jawfish or gobies, don’t necessarily grow very large. Even as adults, they’re reasonable. But the deep sand beds they tunnel through take up TONS of room. And even if you think your aquarium can accommodate both species, you may not have room.

Aquarium stocking guides – proper ones, anyway – focus on so much more than the size of the species. And that’s the frame of mind you want.

Incompatibility of Some Species Regardless of How Many Gallons

All of the tank room in the world may not be enough if you accidentally combine mortal enemies. You need to look at whether your species are compatible when you consider your aquarium stocking guide. Will two fish tolerate sharing a tank together? Or will you end up with fish warfare?

For instance, it’s reasonable – in terms of stocking – to set up a nano aquarium with a royal gramma and a royal dottyback. They don’t need tons of room, and the smaller tanks should do the trick. But if you settle on that combination, you’ll end up with the ultimate fighting championship in your living room. The two species HATE each other, and no amount of space will break up the battles. The same goes for clownfish and damselfish.

Compatibility is a HUGE part of aquarium stocking guides. You need to know whether species play nicely together, how much room they need to live together, and if you need special circumstances. For instance, does setting up a school defuse tensions? Or do you actually WANT to overstock your tank? (It sounds crazy, but it works for some species) When you start researching your favorite species, you’ll usually find this information in their care guides.

Then it’s simply a matter of applying it to your aquarium stocking guide.

Aquarium stocking guides consider the activity levels of all of your fish

Aquarium Stocking Guide: Breathing AND Swimming Room

The old “inches of fish per gallon” rule MIGHT work – if fish never moved. However, fish and invertebrates remain active throughout the day. And every species has its own daily routine. So when you’re looking at an aquarium stocking guide, you need to keep those behaviors in the back of your mind.

  • Does the species hang around the live rocks?
  • Is your fish a constant swimmer?
  • Should you set aside sand for burrowing?
  • Do you need to have structures for exploration?

All of those questions go into deciding how much space you have in your tank for fish. For example, hawkfish stick close to rockwork. But tangs zoom around like perpetual motion machines. Your hawkfish will happily dwell in a tank around 40 gallons (150L) – provided you offer the right environment. But if you find a tang of the same size and try to set it up in the same aquarium, you’ll face extreme aggression (assuming it survives). Tangs need a minimum space of 125 gallons (473L). Everything depends on the behavior of the species.

Aquarium stocking guides will help you by looking at these activity patterns. Active fish? They need BIG tanks with open water. The same with burrowing fish, as they need to construct their dens. Algae grazers usually allow you to scale down a bit. Perching fish won’t ask for a ton of room. And if you want to get the best, you can combine one (or more) of each type. But you need to take the space concerns of each into consideration.

This is why modern aquarium stocking guides got rid of that “inches of fish” rule. It simply didn’t work. You can’t measure behavior. And trying to assign a numerical value to aggression? Yeah, not so effective. You want to take EVERYTHING into consideration and THEN decide how many fish you can stock in your tank.

The Other “Fish”

Of course, saltwater aquariums require an additional step for aquarium stocking guides. You usually want to keep corals and other invertebrates in your tank. So you also have to keep in mind room for THOSE animals. While corals don’t move, they have sweeper tentacles and polyps that require room in the tank. And if you don’t provide enough space, you can end up with problems. The same for your crustaceans, mollusks, or gastropods. Every creature in your tank needs to factor into your calculation.

Every living thing added to your reef tank will add waste products. And that’s what the aquarium stocking guide helps you determine. So even if you’re considering a snail or clam, look at the space they need and make sure your aquarium has the room.

Always consider the compatibility of species when looking at aquarium stocking guides

Proceed With Caution

Once you have an aquarium stocking guide mentality, it’s tempting to hit the fish store. You know how many of each type of fish your tank can hold, and you’re ready to get things set up. But you need to take things SLOW. Oh, sure, you have a 70-gallon (265L) tank that can hold 37 fish (a rough estimate here). But if you added all of them simultaneously, you’ll destabilize the biological filter and end up with a bunch of dead fish. That ISN’T what you were going for. (At least, I’m guessing you weren’t)

NO aquarium stocking guide plans for you to add the entire biological weight of the tank at one time. You need to take your time and allow the tank to cycle after each new addition. Once your water parameters settle back to normal, THEN you can move on to your next purchase. If you don’t take a gradual approach with your stocking, you end up adding too much waste into the system. Even the best filters can’t cope. And then all of your careful research and calculations amount to nothing.

There’s no need to rush things. Take your time and research one species at a time. You’ll also give your fish a chance to adjust to one another. This can help solve any compatibility issues you might have missed when looking at aquarium stocking guides. And it’ll save you aggravation (to say nothing of a hit on your bank account) down the road.

For More Information

Aquarium stocking guides have progressed from the old “measure the fish” rule. Now you need to do more homework. But hobbyists end up with healthier and more beautiful tanks out of the extra work. And if you still aren’t sure, just check out this additional information.

Specific recommendations for saltwater fish by aquarium tank size

Now that we have discussed the basic principles behind stocking your aquarium with the right species and number of saltwater fish, check out these tank builds that include specific recommendations based on the size of the tanks:

Saltwater fish tank builds (including stocking numbers) for: 

To dive deeper into stocking guide strategies

Let’s start with this YouTube video, providing an aquarium stocking guide and strategies on choosing your fish species:

If you still want a rough idea of the number of fish you’re looking at, you can always turn to a handy calculator for answers. (It’s not recommended to rely on this method, though. Aquarium stocking guides work MUCH better)

Interested in some of those SUPER active swimmers? If you’ve got the tank space, we’ve got the species:

Or maybe you’re looking for fish in that mid-range region

Conclusion- How many fish per gallon?

So how many fish should you keep in your aquarium? There’s no definitive answer to that. It depends on all of the elements in an aquarium stocking guide. Sure, it’d be great if a rule like “an inch/2.5cm a gallon/liter” worked, but it doesn’t. You need to look at the compatibility, behaviors, and daily routine of the species you’re planning to keep. And then you need to make sure you can maintain your tank’s water conditions and meet the husbandry needs of your fish and invertebrates.

Suppose you set up your aquarium with SERIOUS filtration and are fanatic about doing water changes. In that case, you can afford to stock a little heavier than an aquarist (like me) who doesn’t have as elaborate a maintenance system. But if you have any doubts, it’s best to stay on the low side of things and give your fish plenty of room.





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