I know people who do research on aquarium fish in the wild. Overfishing and pollution effects on valuable stocks are very popular right now. And research about aquaculture and how sustainable it might/might not be.
Actually, those are not exactly aquarium careers... But there are tons of options in the study of tropical fish in general!
As far as aquarium careers go, I think the most successful are the private businesses that take care of tanks in my dentist's office
my cousin actually maintains aquariums for very wealthy folks, who want gigantic saltwater tanks in their living rooms and stuff.
Working at government-funded zoos and aquariums involve conservation, research, outreach and all that good stuff along with maintenance of tanks. Biiigggg tanks. Full of very expensive or very rare species!