My guess would be that if you glossed the fish, you should use at LEAST a semi-gloss poly finish, but I think I'd try the full gloss first. *If it doesn't look right, mask off your fish and hit the habitat portion with the semi-gloss. Probably best to maintain one finish on the whole project.
I've always finished my habitat displays in either a semi or satin finish. *That's because, in the water, fish do not have that wet slimy look. *That doesn't happen till a fish is out of the water and the mucuse coating reflects the light. *Back in the water, the fish just look colored (satin, in my opinion).
That high gloss finish, so popular with carvers, is probably the ressult of the type of finish used by taxidermists. *Most of their mounts were on display plaques, and were presented as they were caught and taken out of the water. *I think this was carried over to the displays in habitat, too. *
Another site you might check for advice is
http://www.fishcarver.com * There is a forum there dealing only with fish and habitat, by one of the best, Ed Wallicki.
Last week I checked with the gallery owner, where several of my fish are, and asked her about that gloss/satin debate. *She told me that both styles are used and some prefer it one way and some the other, but she agreed with me about the shiny appearance ONLY being after a fish is taken out of the water.
I've snorkled quite a bit doing observations of the fish I carve, and that is where I got my basics on the slippery look.
Al