Saltwater aquarium fish

Decide early whether corals are in your future, because it changes which fish you can keep. Species that nip coral polyps are fine in a fish-only tank and a problem in a reef. Every saltwater profile here, apart from three brackish fish, gives a reef safe rating you can check before you buy.

A fish-only tank with live rock (FOWLR) is the easiest way in: the rock carries the biological filtration and you skip coral lighting entirely. It is a legitimate way to run a marine tank, not a compromise.

Plenty of saltwater fish are sold as juveniles at a fraction of adult size. A miniatus grouper is a 3 inch fish in the store and a 17.7 inch (45 cm) predator a few years later. It needs 180 gallons (680 liters) as an adult. Scats and lionfish grow the same way. Every profile lists adult size and minimum tank size, so check both against the tank you actually own.

A 20 to 30 gallon (75 to 115 liter) marine tank holds fewer fish than the same tank of freshwater, because most reef fish hold territory and the water has less margin. In 30 gallons a full stocking looks like this: a pair of ocellaris or percula clownfish, one small basslet or dottyback (a royal gramma or an orchid dottyback, not both, since they fight over the same holes), one goby or blenny for the rock and sand, and a pair of cardinalfish. In 20 gallons, drop the cardinalfish. The royal gramma and orchid dottyback list 29 gallons (110 liters). Springer's, magenta and diadem dottybacks list 20 gallons (75 liters) and can take that slot. That is the tank. What does not fit: any tang, any of the larger angelfish, and any active swimmer over 6 inches (15 cm). The six-line wrasse, fourline wrasse and twospot hogfish are the only wrasses that fit, and only in 30 gallons. The "fits a 20 gallon" and "fits a 30 gallon" rows above are computed from the catalog and are the honest shortlist.

Most reef fish hold a patch of rock and want nothing to do with a crowd, so a real school is rarer in a marine tank than in a freshwater one. The exceptions are worth knowing. Blue-green and green chromis hang in a loose group in open water above the rock, the cardinalfish drift together over a coral head and come alive at dusk, and the pyramid butterflyfish and schooling bannerfish shoal the way they do on a reef wall once the tank is big enough. Keep chromis in a group of six or more, because a pair or a trio thins out as the strongest fish picks on the rest. Cardinalfish are calmer, and a group of yellow cardinalfish starts at 20 gallons (75 liters). Most other cardinalfish list 29 to 30 gallons (110 to 115 liters).

The marine giants are on this list too, and they are the ones a new reefer buys at 3 inches and regrets at 12. The groupers and unicornfish all pass 16 inches (40 cm). The large angelfish reach 11.8 to 19.7 inches (30 to 50 cm). The morays run from 11.8 inches (30 cm) in the golden dwarf moray to close to 6 feet (180 cm) in the honeycomb moray. Many of these fish are not reef safe either, which is why they sit on the FOWLR list as well. Read the adult size and the tank size on the profile before you buy, and expect nearly all of them to want more than 100 gallons (380 liters).