Also called tumor, tumour, growth, cyst, neoplasia
Unknown for now.
Lumps can be true tumours (some genetic, e.g. melanoma in platy x swordtail hybrids and fibromas/sarcomas in goldfish; some virus-linked, e.g. lip fibromas in angelfish), gonadal tumours (koi) that swell the belly, iridophoromas in bettas, or the viral disease lymphocystis (cauliflower-like growths, usually on fins). A vet diagnosis (biopsy, ultrasound, histology) is needed to tell them apart.
See a fish vet for diagnosis. Some tumours can be treated surgically: fish that are not badly debilitated are good candidates for removal of gonadal masses, and debulking lip fibromas lets fish feed again. Lymphocystis is usually self-limiting.
If a fish is badly ill, talk to an aquatic vet.
Unknown for now.
3 facts checked with sources
0 species