Hi! I posted in the bug/glitch forum, and I saw your problem. Waaay back before my break on Gaia I remember having that problem with my aquarium once. Do you maybe have the Aquarium Jaws in your tank? I don't know, but if it IS a fish, did you ever put it in your tank? I'm pretty sure if you put it in your tank, you can not sell it.
Sorry if this turned out to be a useless comment!! sweatdrop