I don't know if I can really rationalize how I feel about GMO's, but I know that to me it just intuitively feels wrong. And there's a big difference between keeping a pet, and toying with its genetic structure.
As for what Rob said, I don't know if there is a moral difference, but I there *is* a difference. In the case of selective breeding, the traits already occured naturally in at least one member of a species. It's crazy things like taking the web spinning abilities of a spider and adding it to a goat so that the goat grows hair as strong as spider's web, so that bullet proof vets can be woven out of it (I know it sounds insane, but I have actually heard that people are actually trying to do this... I don't even know if it would be possible...) that bother me. Not because we are modifying organisms for our own pleasure or comfort - that's a whole other story - but just that we don't know what could happen by doing all this. I just get this feeling that if we keep messing with things like this, eventually someone's going to really mess up and something bad and irreversible will happen because of it... I don't really have a problem with these specific fish per se, but more with what could happen if we continue at this rate.