Best saltwater fish for a 40 gallon tank

Best Saltwater Fish for a 40 gallon Tank

A 40-gallon saltwater tank is not too big and not too small. It might actually be just right. It won’t be large enough for some of the open water swimmers, but it will certainly give you plenty of room to create a gorgeous reef tank. Let’s explore some of the best saltwater fish for a 40-gallon tank, so you can see for yourself.

Typical 40-gallon tank dimensions

There are two popular 40-gallon saltwater tank designs:

  • 40-gallon “breeder” has the widest of the two footprints: 36 in. long x 18 in. wide x 16 in. deep. It is 6 inches longer and wider than the 20-gallon long, and the same height as the standard 10 and 20-gallon long. The design here provides a lot of surface area for fish to spread out and define their territories.
  • 40-gallon long is a tall tank that is 48 in. long x 12 in. wide x 16 in. high – that’s the same length and width as the 20-gallon long aquarium, but with an additional 6-inches in height. It is as long as the larger 55 and 75-gallon tanks, but not as wide or deep. 
  Length Width Height
40 gallon breeder 36 in. (91.4 cm) 18 in. (45.7 cm) 16 in. (40.6 cm)
40 gallon long 48 in. (121.9 cm) 12 in. (30.5 cm) 16 in. (40.6 cm)
40-gallon breeder tank shown here

Related: 40 gallon fish tank dimensions

If you are interested in exploring other tanks that share one of these similar dimensions (length, width, or height), here are a few other options:

Other tanks with the same length

  • For a 36 inch length, check out: 30-gallon breeder
  • For a 48 inch length, you could scale up to a 55-gallon tank to a 70 or even 90-gallon aquarium, all with a 4-foot length

Other tanks with the same width

  • 20-gallon and 29-gallon saltwater tanks are also 12 inches wide

Other tanks with the same height

  • 20-gallon “high” tank is also 18 inches high/deep

Looking for something a little bigger or smaller?

Check out the dimensions and stocking guides for other sizes:

Selecting the best saltwater fish for a 40-gallon tank

A 40-gallon aquarium is large enough to care for any of the fish types that are appropriate for a 10, 20, or 30-gallon aquarium, plus a few more. 

The best saltwater fish for a 40-gallon tank share a few key aspects:

  • Peaceful community fish—they swim well with others 
  • Small-to-medium-sized by saltwater tank standards (probably less than 5-inches as adults)
  • Like to perch and/or swim slowly—a few are open water swimmers but are small in size

Recommended 45 best saltwater fish for a 40-gallon tank (fish types and stocking levels)

For a 40-gallon tank, you can add 9-15 fish from the list below. There is no magic formula there—the goal is to pick fish species that will be well suited for the size tank, complimentary with each other, which also generally means they look different and occupy different spaces, and enough of them to fill out the tank and make it look amazing and full, without over-filling.

If your goal is to keep some harder-to-care-for coral types, you will want to stay on the low side of the recommended fish stocking level so that you can keep the nutrient load in the water lower and easier to control. If your primary goal is caring for fish and perhaps some hardy beginner corals, you can stock on the higher end. Of course, stop when you have achieved the balance you’re looking for. 

A 40-gallon saltwater tank has enough room for the larger clownfish species

Here is a list of 45 best saltwater fish for a 40-gallon tank:

If you have a deep sand bed, you could consider:

Here’s the same information listed as a table below:

Filefish (1) Aiptasia-eating            
Blenny (1) Tailspot 2 Spot Canary Midas Striped Harptail  
Cardinalfish (1) Banggai Yellow Threadfin Pajama Longfin    
Cleaner goby (1) Neon Yellow line Shark nose Hybrid      
Clownfish (1-2) Maroon Clarkii Spotcinctus Ocellaris Percula Orange skunk Pink skunk
Alt to clown (1) Green Chromis            
Dartfish (1) Firefish Purple firefish Zebra Scissortail Helfrichi    
Dottyback (1) not with basslet Orchid Neon          
Fairy Basslet (1) not with dottyback Royal gramma Blackcap Chalk bass        
Fancy goby (1) Clown Two spot Yasha Court jester      
Hawkfish (1) Flame Longnose Falco        
Saddle Valenti puffer (1)              
Engineer goby (1-2)              
Jawfish (1-2)              

Damselfish-only 40-gallon saltwater aquarium

Damsels are colorful, hardy, and inexpensive. They are also wildly popular and available at many fish stores—likely for those very reasons. I have a love-hate relationship with them. I love their colors, they are almost ideal community fish, but my personal experience with them (several different species) is that they were jerks in my community tank.

So my own personal approach is to include them here—because they are some of the best saltwater fish for a 40-gallon tank, but I recommend you keep them “African cichlid style.” By that, I mean, keep them in a tank dedicated to damsels and let them spar with each other. When you keep aggressive fish together with enough fish to spread out the aggression—the system balances. 

If you love damsels, consider a damsel-only tank. Approximate 9-12 damsels in a 40-gallon tank would be a beautiful display. Here are a few great options:

Three perfect saltwater fish for a 40-gallon tank damselfish-only aquarium

Equipment needed for a 40-gallon saltwater tank

To care for the saltwater fish in your 40-gallon tank, you will need the following equipment:

  • A 40-gallon saltwater tank – throughout the year, the standard rectangular tanks cost somewhere around $2 per gallon or more, but many stores run $ per gallon sales (or 50% off, which nets the same thing) if you want to score a bargain
  • A lid or mesh net to prevent fish from jumping out when startled—I suppose you don’t need this to get started, but please be warned it is an unfortunately common event
  • An air pump & airline tubing or a Powerhead pump, like the Vortech, for water flow.
  • Either sponge, hang-on-back, canister, or Live Rock to help with biological filtration
  • 200 Watt heater for most locations- for extra protection against a heater failure, pair it with an ink bird controller
  • A reef tank quality LED light, like Aqua Illumination Prime 16, Kessil, or even the ViparSpectra light are necessary for corals, clams and anemones
  • You can afford to spend less on lights if you are starting with fish only—and upgrade when you are ready. Examples of less expensive (and less bright) lights are these units by AQUANEAT, HYGGER, and NICREW.
    • For example, I use these on my QT tank, over my sump and for a small tank where I’m trying to establish a breeding pair of Banggai cardinals

Some other important topics

Hopefully, this article got you started with a solid approach to planning for the best saltwater fish for a 40-gallon tank and I wish you great success on your journey here. I hope you enjoy this hobby as much as I do.

But wait, there’s more! A whole lot more. Here are a few other articles you should read or re-read as you start planning your 40 gallon tank build.

Going beyond fish

For me—the saltwater tank will always be about the fish…but they are not the only thing. Part of the fun is mixing fish and invertebrates to make a mini-reef ecosystem. Here are a few additional articles to get started thinking about some invertebrates for your 40-gallon saltwater tank:

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