First off, I did write this in one night with minimal research, so I'm not exactly surprised at all of my errors. Also, I think I posted the slightly-less-polished draft, not the one I printed and turned in. Oops.
Anyways, Paragraph 1: I like the idea of the ice minerals as a dietary supplement. IIRC, Europa does have some geysers, just not as many as Enceladus. I'll look into hydrothermal vents and algae.
Paragraph 2: Yeah, I really just estimated most of my numbers. The idea is, their body temperature is extremely high, so if a predator or something developed more of a dependence on thermal vision, bye-bye fishies. Honestly, this aspect is probably going to go through a lot of revision before I'm happy.
Lastly, thanks for your feedback! Who knows, I might write them into a story. Heck, I might just let you have them if I never get around to doing anything. We'll see how it all works out.