A thorough understanding of Freshman level Biology/Chemistry would do many, many people in this hobby a lot of good. If you're looking to learn pertinent useful information, those classes and their accompanying texts, practicals etc. if taken seriously and indulged fully, can be incredibly useful.
If you want to take it a bit further, microbiology, vertebrate anatomy, ichthyology, are all good places to look.
If you want to look at pretty fish and pick up a few tidbits of info, I recommend a nice picture book.