Smallest Fish: The 10 Tiniest Species and Smallest Pets

By the usual measure, Paedocypris progenetica, a peat-swamp fish from Sumatra, Bintan and Borneo whose females mature at 7.9 mm and which is not known to exceed about 10 to 11 mm (Wikipedia). The answer changes with the measure, and Wikipedia's list is explicit about it:

• Length at maturity: Paedocypris progenetica, 7.9 mm for females
• Maximum size: Paedocypris, not known to exceed 11 mm, level with the smallest gobies
• Weight: the stout infantfish, Schindleria brevipinguis, at under 1 milligram
• Males only: the anglerfish Photocorynus spiniceps, whose males mature at 6.2 mm but live attached to females that reach 50.2 mm

Paedocypris was described in 2006 by Maurice Kottelat and colleagues. It lives only in acidic water, has a partly cartilaginous skull and was called the smallest vertebrate until a frog, Paedophryne amauensis, was described in 2012 (Wikipedia).

Wikipedia's list of smallest fish has ten species, given here with size at maturity and maximum size. Only a few specimens of some have been measured, so the order can shift.

• Paedocypris progenetica (Cyprinidae): 7.9 mm at maturity (female), 10.3 mm maximum
• Photocorynus spiniceps (anglerfish): 6.2 mm (male), females to 50.2 mm
• Schindleria brevipinguis, stout infantfish (Gobiidae): 6.5 mm (male), 10 mm
• Pandaka pygmaea, dwarf pygmy goby (Gobiidae): 9 mm (male), 11 mm
• Trimmatom nanus, midget dwarfgoby (Gobiidae): 10 mm, 10 mm
• Danionella translucida (Cyprinidae): 10 mm, 15 mm
• Danionella cerebrum (Cyprinidae): 10 mm, 12.6 mm
• Hyaloglanis pulex, a catfish (Trichomycteridae): 10 mm, 14.9 mm
• Leptophilypnion fittkaui (Eleotridae): maturity size unknown, 9.5 mm
• Leptophilypnion pusillus (Eleotridae): maturity size unknown, 9.1 mm

The pattern is clear: most are gobies, sleeper gobies or miniature carps from Southeast Asia and the Amazon.

The dwarf pygmy goby, Pandaka pygmaea. Males mature at 0.9 cm and reach 1.1 cm in standard length, females reach 1.5 cm in total length, and adults weigh about 425 milligrams (Wikipedia). It was described by Herre in 1927 from Malabon in Metro Manila, where it was later wiped out by land reclamation; it now survives in places including Culion Island, Bali, Sulawesi and Singapore. It lives in shallow brackish water and mangroves.

The Philippines also has the sinarapan, Mistichthys luzonensis, of Lake Buhi and nearby lakes in Camarines Sur and Albay. It grows only to 2.5 cm, is rated Vulnerable, and holds the Guinness World Records title of smallest commercially harvested food fish (Wikipedia). Overfishing and introduced Nile tilapia pushed it close to disappearing by the 1990s.

Among fish commonly sold for home aquariums, the chili rasbora, Boraras brigittae, is one of the smallest: usually 10 to 20 mm, with the largest recorded at 3.1 cm (Wikipedia). It comes from peat-swamp blackwater in southwest Borneo, is red to orange with a black stripe, eats tiny insects, worms and crustaceans, and can live up to 8 years with good care. The dwarf pygmy goby has also reached hobbyists; Wikipedia records it being imported to Germany through the aquarium trade in 1958.

The record holders themselves are not pets. Paedocypris needs acidic peat-swamp water and is threatened by swamp drainage and fires (Wikipedia). Tiny fish also need stable water more than a big tank does: see the betta fish care notes and keep nano species in groups in a planted, filtered tank.

Young fish have stage names (Wikipedia, Juvenile fish):

• Larva: just hatched, still living on a yolk sac
• Fry: yolk nearly used up, swimming and starting to feed
• Fingerling: fins and scales formed, typically about the size of a human finger
• Juvenile: adult-shaped but not yet mature
• Salmon stages: alevin, parr and smolt
• Food trade: whitebait is the fry of herring, sprat, sardine and mackerel, 25 to 50 mm long; young eels are glass eels and elvers

A small adult fish is a different thing: it is a species that stays small, like the ones above. If you are not sure whether a tiny fish is a baby or a small species, count the fins and look for adult colour; the FishLens app can also check a photo.