20 gallons (75.7 liters) can be a great size for a saltwater tank. It is small enough to fit into most rooms or apartments, yet not so large that it takes up a hulking amount of space.
20 Gallon tank dimensions
There are a few different designs/shapes available in this size. There are two primary options that will be available at almost any pet store you find:
- 20L – The “L” designation, in this case, stands for “Long”, not “Liters” – the traditional dimensions are 30 in. x 12 in. x 12 in. ( 76.2 cm x 30.5 cm x 30.5 cm) which is 6 inches longer than the model below
- 20H – The “H” designation stands for “High” — the dimensions are 24 in. x 12 in. x 16 in. (61 cm x 30.5 cm x 40.6 cm), which is 4 inches taller than the previous model
| Tank | Length | Depth | Height |
| 20-L | 30 in. (76.2 cm) | 12 in. (30.5 cm) | 12 in. (30.5 cm) |
| 20-H | 24 in. (61 cm) | 12 in. (30.5 cm) | 16 in. (40.6 cm) |
24 inches x 12 inches x 16 inches
Ruling out saltwater fish for a 20-gallon tank that are a bad fit
To select the best saltwater fish for a 20-gallon tank, let’s first rule out a few options that are not a great fit.
No open-water swimmers
The most active, open-water swimmers are not suitable for a fish tank with these dimensions–there just isn’t enough room for them to be happy. That means no tangs, angelfish, butterflyfish, anthias, wrasses. 20-gallons just doesn’t provide enough room for these great fish.
No fish larger than 4 in. (10 cm) in length
In a 20-gallon tank, you also likely want to limit your options to fish that are about 4 inches in length as adults, or less. That rules out rabbitfish, triggerfish, eels, pufferfish, certain larger clownfish species (Maroon, Clarkii, Tomato, etc.).
No fish that require expert-level experience
There are a few gorgeous saltwater fish, like Mandarinfish and Scooter Blenny, that meet our other criteria but should be ruled out for the vast majority of aquarium owners because they are notoriously challenging to care for.
The best saltwater fish for a 20-gallon tank
The best saltwater fish for a 20-gallon tank:
- Small in size (4-inches or smaller as adults)
- Naturally live in relatively smaller territories (site attached)
- Display slow, graceful swimming or perching behaviors
These 8 options also have similarly sized alternatives you could choose
26 Recommended saltwater fish for a 20-gallon tank (fish types and stocking levels)
- 2 small Clownfish ( Ocellaris, Percula, Orange skunk, or Pink skunk)
- 1 Dartfish (Firefish, Purple firefish, Zebra, or Scissortail)
- 1 Fancy goby (Clown, Two spot, Yasha or Court jester)
- 1 Cleaner goby (Neon, Yellow line, Shark nose, or Hybrid)
- 1 Small blenny (Tailspot, Two spot)
You could also choose from this next list of fish that are a bit larger, but should also be alright in most 20-gallon tanks:
Choose from the fish options below:
| Clownfish (1 pair) | Ocellaris | Percula | Orange skunk | Pink skunk |
| Dartfish (1) | Firefish | Purple Firefish | Zebra | Scissortail |
| Fancy goby (1) | Clown | Two Spot goby | Yasha | Court Jester |
| Cleaner goby (1) | Neon | Yellow line | Shark Nose | Hybrid |
| Small blenny (1) | Tailspot | Two Spot blenny | ||
| Cardinalfish (1) | Banggai | Yellow | Flame | |
| Hawkfish (1) | Flame | Longnose | Falco | |
| Fairy basslet (1) | Royal gramma | Blackcap |
A damselfish-only 20-gallon saltwater tank: a great alternative to a community tank
Another great list of saltwater fish to choose from are the damselfish. Damselfish are some of the most popular and inexpensive saltwater fish available. The biggest problem is that they are territorial and aggressive. But one of the ways you can safely add aggressive fish into an aquarium is to limit your options to all aggressive fish.
I do not advise you to mix damselfish with the community fish in the recommended list above, but you could create a tank that is dedicated to these small jewels of the reef.
To create your own damselfish tank, you could pick 5-7 of the following damsels:
Equipment needed for a 20-gallon saltwater tank
To care for the saltwater fish in your 20-gallon tank, you will need the following equipment:
- Lid
- To prevent fish from jumping out when startled–believe it or not, this happens A LOT
- Water flow and aeration
- Filter
- Heater
- Lights
- For a fish-only aquarium, just about any aquarium light will work. You won’t need an expensive reef light
A few important related topics to consider
Now that you are thinking about which of the best saltwater fish to add to your 20-gallon tank, here are a few other articles related to the topic of adding saltwater fish to your tank that will help you on your journey towards creating a successful aquarium:
Other stocking options
This article focused on the best saltwater fish for a 20-gallon tank, but there are lots of other great invertebrates and corals you could choose from. If you’re interested in corals, crabs, shrimp, check out these other great articles for some inspiration:
Best beginner corals
Cleaner shrimp
