Skip to main content
Saltwater Fish

Best Saltwater Fish For 20 Gallon Tank

By December 7, 2021No Comments
saltwater fish for 20 gallon tank

One of the most common sizes of aquariums is 20 gallons. This size saltwater tank is affordable and easy to obtain. Most pet stores carry these.

A lot of fish are large and not suitable for a smaller tank like this, so it is important to understand which saltwater fish are best for a 20 gallon marine tank. This goes for reef tanks as well.

Nano tanks come in different sizes, and it is important to understand the best choices based on how much saltwater your tank holds.

What Makes A Saltwater Fish Good For Nano Sized Tanks?

Smaller fish and less active ones are the best choices for nano sized tanks. Also, most people use size of fish to determine how many to keep, not total number. You may be looking for 3 fish for your saltwater tank, but you certainly could not do a Naso Tang, a Watchman Goby, and a Koran Angelfish as your 3. The size of these fish would be overkill, and that is not even considering the amount of waste they would put in such a small tank.

Stick to smaller sized fish for a total of about 6-9 inches total. An example of this would be 2 clowns and a watchman goby.

20 Gallon Tank Sizes

Here are the average sizes for tanks that are 20 gallons:

  • 20 Gallon High: 24″ x 12″ x 16″
  • 20 Gallon Long: 30″ x 12″ x 12″

8 Best Saltwater Fish For A 20 Gallon Aquarium

ClownfishClownfish3 inches
Yellow Watchman GobyGoby3.5 inches
FirefishDartfish3 inches
Wheeler's Shrimp GobyGoby3 inches
JawfishJawfish3.5 inches
Court Jester GobyGoby3 inches
Smith's BlennyBlenny3 inches

1. Clownfish


Clownfish are by far the most popular fish for almost all size tanks, including nano tanks. When you have them in your tank, everyone will know that you have a saltwater tank. A lot of people cant tell the difference between saltwater and freshwater, but once they see clowns, they will know.

The most common type is (Amphiprion ocellaris). These look like Nemo in Finding Nemo. The good news is that are a lot of varieties of clowns. I keep a pair of chocolate clownfish in my nano tank. I already had Oscellaris in my main tank, so I wanted something different looking.

It is recommended to keep them as a pair, so you are looking at 6 inches of fish total if you get clowns. This means that you are looking at adding one more fish to a 20 gallon if you get a pair of these.

2. Yellow Watchman Goby

yellow watchman goby

The Yellow Watchman Goby is a great choice for smaller aquariums due to its behavior. These gobies will mostly stay on the sand bed, and they will claim a cave in your live rocks. This will be their home. Most fish stores sell smaller sized ones, but they can get big at maturity (3.5 to 4 inches.) The smaller sizes are perfect for nano tanks, because they can be hard to find in larger tanks.

These fish also have a unique frown, so being able to see them up close is a plus.

These gobies will form a symbiotic relationship with Pistol Shrimps. If you really want an interesting 20 gallon marine tank, then pair this goby up with a Pistol Shrimp, and you have a great tank already.

3. Firefish

Rickard Zerpe, CC BY 2.0, via Wikimedia Commons

The Firefish is a unique fish, and it has beautiful colors. If you want something in your tank that stands out, then consider this one. They are not active swimmers, and then tend to float in one area. Mine likes to swim front and center of my tank. When they are not out, they do like to hide in live rocks. Keep in mind that this fish will not always be out and swimming like a Clownfish would. They stay small and will only get to 3 inches at maturity which is a perfect size fish for nano sized tanks.

4. Wheeler’s Shrimp Goby

Wheeler's Goby
Carl Malamud, CC BY 2.0, via Wikimedia Commons

This Goby will hang out on your sand bed, and it is fun to watch it. It has nice striped coloration along with the Goby frown. I like the Yellow version better, but they are both great for small tanks. It is peaceful, and will not bother anything else in your tank.

5. Blue Dot Jawfish

jawfish in aquarium
Rhododendrites, CC BY-SA 4.0, via Wikimedia Commons

If you are looking for an expensive fish for your nano, then look no further than the Jawfish (Opistognathus rosenblatti). You will likely be paying around $200 for one of these. Just because you have a smaller aquarium, does not mean you cannot keep expensive livestock in it.

It may be pushing it just a bit to include these for anything under 30 gallons, but if you have the right setup, you should be fine. You will need plenty of live rock, small substrate (sand) for it to sift through, and you will need a lid on your tank. These fish have known to be jumpers. How bad would it be to spend the cash on one of these only to find it on the floor the next morning. Get a lid for your tank.

6. Court Jester Goby

court jester goby in 20 gallon nano tank
Tappinen, CC BY-SA 3.0, via Wikimedia Commons

The Court Jester Goby (Koumansetta rainfordi) is colorful and very fun to watch in any aquarium. I have had one of these in my nano tank, and I enjoyed it. They swim about and sift sand to find food. Whenever you have a tank with only a few fish in it, I like to have species that stand out or have interesting traits. This guy fits the bill because of the way it looks for food by playing in the sand.

If you have no sand in your tank, this will not be a suitable fish for you.

The photo above does not do this fish justice, as its beautiful orange stripes stand out way more than shown here.

7. Smith’s Blenny

disco blennie in 20 gallon aquarium
Rainer Kretzberg, CC BY 3.0, via Wikimedia Commons

Smith’s Blenny or Disco Blenny (Meiacanthus smithii) likes to hang around live rocks and will pick at food etc. They are an interesting fish to watch, and they seem to be always moving about. While it may not have the most colorful look, it is still a nice choice for tanks of this size.

20 Gallon Saltwater Tank Stocking

You can certainly make a saltwater tank in a 20 gallon tank. But, how many fish do you need to stock it? 3 to 4 fish is a great number to reach for an aquarium of that size. If you go higher than that, you will run the risk of excess waste and nutrients. This will lead to a dirty and unhealthy tank.

One of the most popular nano fish are clownfish. They also do great in pairs. A good starting point to stock a tank this size would be a pair of clownfish and one more fish. This could be a goby, wrasse, or any other fish on our list.

It is also a good idea not to stock too many of the same type in such a small tank. An example of this would be putting 3 rock dwellers in your tank. Territorial fish that cling to live rocks like gobies will not do well in tight spaces with each other. So avoid that situation.

Saltwater Fish To Avoid For 20 Gallon Tanks

There are so many beautiful and large marine fish that look great, but not all of them are suitable for smaller aquariums. If they get over 4 inches, they are probably not a good choice.

Here is a list of some of the most common larger fish that you should not keep in small tanks.

  • Tangs
  • Angelfish
  • Rabbitfish

How Many Fish For A 20 Gallon Reef Tank

Stocking Guidelines for saltwater fish in a 20 gallon tank is 2 to 3 fish as long as they are no more than 3 inches long each. This will help you keep a cleaner aquarium, and it will keep unwanted nutrient levels low.


My name is James, and I am the founder of Saltwatercoraltank. I love everything about the ocean, and my main hobby is saltwater aquariums. Currently, I have 3 tanks that I maintain. I have a 130 gallon mixed as my main, and I prefer softies.

Leave a Reply