I would call local vets. Explain it's a fish and obviously you can't bring it in on a leash. You need a jar of formalin from the lab (they are supplied free to the vet). Excise the mass as planned and take jar with mass to the vet. They will send it to the lab. You want a biopsy report with histopathology. Should take about 3 days for results. Cost could be quite variable for the same thing, so call around. Usually more rural is cheaper. Here the fee at clinics I work at ranges from about 100-250 (for the same report).Where would I go about getting it biopsied? I'm in the northeast outside of NYC and I imagine theres not too many places that deal with fish.