Experiment with lots of different styles of tank and eco-system, as well as different fish species. Do you have an aquarium near you, or a zoo with an aquarium section? One of my friends volunteers there as a fish feeder. It means she gets to feed giant groupers and cave nurse sharks.
Read lots, about both fresh and saltwater species.

As well as the courses Olympia has mentioned, do keep up with English as well. Employers are always more impressed with excellent spoken and written skills, and as ichthyology will no doubt land you in a research position, it will stand you in good stead for writing papers and reports.
