What makes reef fish compatible
Compatibility is mostly about five things: temperament, adult size, territory, diet and where in the tank each fish lives. Two fish can both be labeled “peaceful” and still fight if they compete for the same cave or share a similar shape and color. Looking at each factor before buying helps prevent most problems.
- Temperament. Peaceful fish do best with other peaceful fish. Mixing very shy species with boisterous ones can stress the shy fish.
- Adult size. A fish that will eat anything that fits in its mouth is not a good tank mate for small fish, even if it behaves when young.
- Territory. Many dottybacks, damselfish and some gobies defend a home area.
- Diet. Fish with specialized diets, such as dragonets, can be outcompeted by faster eaters.
- Tank level. Combining bottom dwellers, mid-water swimmers and rock-perchers can reduce conflict.
Which fish can stay together
There are no guarantees, but the following combinations are commonly reported to work in appropriately sized tanks:
- Clownfish with cardinalfish, gobies and blennies. These are often considered easy combinations.
- Watchman gobies with pistol shrimp. A well-known partnership in which the shrimp digs and the goby keeps watch. See the best goby for a reef tank.
- Cardinalfish groups with other quiet fish. Pajama and Banggai cardinals are generally peaceful, but Banggai can be territorial among themselves in small tanks.
- Firefish with other gentle fish. Firefish are peaceful but easily intimidated by boisterous species.
A good rule of thumb is to keep fish of similar temperament, avoid fish that look alike (which can trigger territorial fights), and keep only one of any species known to be aggressive with its own kind unless you have a confirmed pair or a large tank.
Good tank mate candidates
Frequently recommended reef-tank species include ocellaris clownfish, dottybacks, gobies, cardinalfish, blennies, firefish, jawfish, basslets and some small wrasses. Many of these are covered in our guide to reef safe saltwater fish and the longer list of best saltwater community fish.
Some other fish need special mention:
- Jawfish dig burrows and may rearrange your sand, so give them a deep sand bed and rock out of reach.
- Dragonets have special diets and are often kept in mature tanks with plenty of live food.
- Dartfish and firefish jump, so keep a tight lid.
Combinations that often cause problems
- Several dottybacks or damselfish in a small tank
- Two of the same species of dwarf angelfish or hawkfish without enough room
- A fast, aggressive eater with a shy, specialized feeder
- Large predators with small fish or shrimp
- Many tangs in a tank that is too small, particularly tangs of similar shape
Which fish eat coral or bother invertebrates?
Fish known to eat or damage corals or invertebrates include many butterflyfish, certain angelfish, triggerfish, puffers, and some wrasses and filefish. Even normally safe fish may nip at clam mantles, zoanthids or large-polyped stony corals, so watch new fish closely. Always check the species’ diet before buying.
Adding fish in a sensible order
- Add the most peaceful and smallest fish first.
- Introduce territorial or semi-aggressive fish last, so they do not claim the whole tank.
- Add fish gradually rather than all at once, to protect biological filtration.
- Rearrange rock slightly when adding a new fish, which can reduce established territories.
- Quarantine each addition. See saltwater fish care for the basics.
Tank size and swimming space
A larger tank gives fish room to avoid each other and spreads out waste. As a general guide, active swimmers such as tangs need long tanks, while nano reef fish like gobies and cardinals can live in smaller setups. If you are unsure, choose fewer fish and species with modest adult sizes. Peaceful tanks are easier to maintain than tanks where you are constantly managing aggression.