Many of the tropical fish seen in pet stores are not fully grown. The final size of the fish varies dramatically between species. This is a list of the sizes that individual species are likely to grow to. Only some species are listed here, and adult fish of the same species vary somewhat in size. This is meant as a general guide to avoid buying fishes that will outgrow your tank.
Some of these species are sometimes illegally wild-caught and exported from their countries of origin; just because it is on this list is not a recommendation for keeping it.
Enormous Tropical Aquarium Fishes (Over 16 in.)
Red Tail Catfish: Phractocephalus hemioliopterus
Arapaima, Pirarucu: Arapaima gigas (potentially to 16 ft. usually less)
Arowana: Osteoglossum bicirrosum, Osteoglossum vandelli
Black Arowana: Osteoglossum ferreirai
Asian Arowana/Green Arowana: Scleropages formosus
Australian Arowana: Scleropages jardini
African Arowana: Heterotis niloticus
Tiger Shovelnose Catfish: Pseudoplatystoma fasciatum
Shovelnose Catfish: Sorubim lima
Reticulated Pimelodid: Perrunichthys perruno
Plecostamus, Pleco, Suckermouth Catfish: Hypostomus plecostomus
Snow King Plecostomus: Liposarcus anisitsi
Blue-eyed Panaque, Blue-eyed Plecostamus: Panaque suttoni
Sailfin Plecostamus: Pterygoplichthys gibbiceps
Walking Catfish, Albino clarias: Clarias batrachus
Fire Eel: Mastacembelus erythrotaenia
Ornate bichir: Polypterus ornatipinnis
Knife Fish; Featherback: Notopterus chitala
Mozambique Mouthbrooder: Oreochromis mossambicus
Gourami: Osphronemus goramy
Black Shark: Labeo chrysophekadion
Sources:
Butler, Rhett A. 1999-2008. Mongabay.com – San Francisco, CA
Practical fish keeping http://www.practicalfishkeeping.co.uk/pfk/pages/home.php
Mills, D. 1987 Illustrated Guide to Aquarium Fishes. Galaxy Press, England
