Top 5 Best Saltwater Fish For a 20 Gallon Tank

As far as aquarium sizes go, a 20 gallon (76 liters) fish tank is one of the smallest and is considered to be a nano-sized aquarium. Nano aquariums are any sized aquarium that is under 30 gallons (113.5 liters). Typically, 20 gallon (76 liters) aquariums come in many different dimensions, with the most popular being 24 inches x 12 inches x 17 inches (60 cm x 30 cm x 43 cm). Another incredibly popular nano fish tank is the 10 gallon saltwater fish tank.

Here are our top 5 best saltwater fish for a 20 gallon tank:

  • Watchman Goby
  • Clownfish
  • Pajama Cardinalfish
  • Royal Gramma Basslet
  • Yellowtail Damsel

Read on to see our in-depth analysis on each of these beautiful fish.

20 gallon saltwater fish tank

The trouble with having a 20 gallon saltwater fish tank is not necessarily acquiring the tank itself, you can find these in abundance in your local fish store or online, it is finding the best saltwater fish for a 20 gallon (76 liters) tank that most people struggle with.

Fortunately, we have devised a comprehensive list of the top 20 gallon saltwater fish to choose from. When picking the best fish for this small tank size, we look at personality, behavior, tank mates, and beauty (which of course, is in the eye of the beholder!).

The Top 5 20 Gallon Saltwater Fish (List)

A common question posed by hobbyists is, how many saltwater fish can I put in a 20 gallon tank? Now, depending on the size of the fish, you should typically only keep between 4 and 5 fish in a 20 gallon tank. This is under the assumption that the fish are a maximum of 3 inches (7.5 cm) in length, if the fish are smaller, then you can add more.

Now, let's dive into our list consisting of the best saltwater fish for a 20 gallon (76 liters) tank. It is important to ensure that if you do not want to introduce any of the fish on this list to your 20 gallon (76 liters) tank that you conduct additional and extensive research on whichever fish you would like to add to your nano aquarium. Also, if you wish to house multiple of the same fish species, you might be better off with a 40 gallon saltwater fish aquarium.

Below is a short video summarizing this article:

1. Watchman Goby

The Yellow Watchman Goby is certainly one of the most unique and mesmerizing fish to enter the list. Despite being known for their peaceful nature towards other fish, these gobies do not play well with others from the same species, unless it is a mated pairing. Keeping 1 goby per tank is our advice.

Watchman goby

These fish possess a neon yellow body with a slight mixture of brown. Typically, the yellow watchman will only grow to 4 inches (10 cm) in length, meaning a 20 gallon tank is more than suitable for them. As far as their requirements go, you will need a sand substrate for them to dig around in, and also housing them with a Pistol Shrimp will benefit both parties tremendously. The goby will let the shrimp know when predators are around as pistol shrimps suffer from poor vision, and in return, the shrimp will help them construct their home and dig around in the substrate.

A varied diet of Mysis and brine shrimp, table shrimp, and frozen meats are perfect for them and they need to be fed at least twice a day. They will also accept high-quality marine foods but it takes them a while to get used to that diet.

2. Clownfish

Clownfish are certainly one of the most popular fish in the hobby, and with 28 different species of clownfish to choose from, the possibilities are truly endless! Out of the 28 different species, some are simply too big to live in a 20 gallon saltwater fish tank. Fortunately, the majority of Clownfish are the perfect size for a 20 gallon tank.

Clowns tend to come in a variety of different colors and markings, with the most commonly seen being orange and white (Nemo). So, how many clownfish can I put in a 20 gallon tank? The most common species of clownfish is the Ocellaris Clownfish (Amphiprion ocellaris), which typically grow to around 3 inches in length. So, if you stick to the 1 inch per gallon rule, you could probably get away with housing a maximum of 6 clownfish in your 20 gallon tank, but we would advise 4 or 5 maximum. It is important to bear in mind too that clownfish groups should consist of all males and one dominant female.

True Percula Clownfish vs Ocellarist Clownfish

Finding clownfish tank mates is a pretty easy task as they tend to get along with most saltwater fish that encompass a peaceful nature. Mixing them with aggressive fish could lead to your clown being bullied which can result in devastating consequences. The relationship between clownfish and anemones is no secret, but most do not require one to survive. Also, anemones can grow pretty big, meaning a larger tank size would be required.

They are not fussy eaters, with a balanced diet consisting of a mixture of plant and meaty foods. Frozen shrimp, algae, bloodworms, saltwater flake foods, and pellets are perfect for them. Additionally, they are reef safe meaning that you can house clownfish with corals without there being any issues. Clownfish became so popular from the Finding Nemo film series that they were actually being taken from wild reefs such as the Great Barrier Reef to sell to saltwater fishkeepers. Fortunately, this stopped and captive breeding is fully in place.

3. Pajama Cardinalfish

The beautiful and incredibly unique Pajama Cardinalfish is a peaceful fish that will thrive within a 20 gallon tank. If you are looking to keep a school of cardinalfish in your 20 gallon tank you will likely run into some trouble as the aquarium is simply not big enough. However, keeping a single fish is perfect, they grow to roughly 2 inches (5 cm) in length and get along very well with equally peaceful tank mates.

Pajama cardinalfish

Pajama Cardinals are typically slow-moving fish, it is certainly advised to keep them with non-aggressive fish that will likely bully them, and also fast erratic fish that will stress them out. They thrive in planted and rocky tank environments as they can hide away in safety. Aesthetically, these fish are pretty unusual, showcasing a variety of different colors, with a green and yellow face and orange striped eyes. Their body is silver with dark black/brown spots. Feeding them a mixed diet of meaty foods like brine shrimp, bloodworms, and saltwater flakes is perfect for them.

4. Royal Gramma Basslet

The Royal Gramma basslet is undoubtedly one of the most beautifully colored fish in the hobby, which is why it found itself on our list for the most colorful saltwater fish. The mixture of bright purple and neon yellow makes them truly mesmerizing marine fish. They are extremely peaceful by nature but are known to not play well with other royal gramma basslets, having one per tank is advised.

Royal gramma basslet

They are very easy to care for and are considered to be incredibly hardy fish. Pristine water conditions are not required for them to survive and thrive in and they adapt very quickly to new environments. Typically, they will grow to around 3 inches (7.5 cm) in size, making them perfect for a 20 gallon saltwater fish tank. By populating your tank with plenty of rocks and hiding spaces you will ensure these royal grammas never get bored!

Royal grammas are carnivores, meaning their diet should consist of a mixture of brine shrimp and Mysis shrimp, with a varied carnivore diet that will keep them healthy and happy.

5. Yellowtail Damsel

The colorful Yellowtail Damsel is a very small fish that thrives in big schools as well as single species. With bright blue bodies and a yellow tail, they will certainly brighten up your saltwater aquarium. Unlike other members of the damselfish species, Yellowtails are mostly peaceful, with some semi-aggressive tendencies.

Yellow damsel

All in all, they are hardy fish that can adapt to many different aquarium conditions and environments. Due to them being omnivores, they will need to be fed frozen and dry meaty foods, including Mysis shrimp. They will willingly accept flake and pellet food too.


Any of the 5 fish that feature on this list would be perfect for a 20 gallon (76 liters) nano tank. However, it is important to be mindful of small tanks occasionally causing some aggression and territorial issues with your fish. Sure, the fish that feature on this list are peaceful by nature, but you can also run into an anomaly.

Moreover, if you wish to house more of the same type of fish, for example the clownfish, you will have to own a 30 gallon tank or more. Provided you pick the right amount of fish, and the right fish, you should be absolutely fine.