Fish in general... BUT, I did spend 3 years researching reef aquariums in my free time (literally going through 20+ books twice, including Julian Sprung's enormous 3 volume set) and would say I have a pretty good knowledge of the basics that translate over to Freshwater.
For instance, routine water changes and the dangers of overstocking as well as maintaining pH levels. The only thing that I am really a tad bit unsure of after reading several books regarding planted aquariums are maintaining the macro and micro nutrients vital to plants...
@LiveHumanSkeleton: Yeah, it's very tempting. Hehe, I think I can manage to withhold for a few more months while I research though.
PS: I think I finally decided on what my tank will have. Hopefully a Southeast asian blackwater biotope featuring various cryptocoryne and dwarf rasboras!!