[Sick fish?]

Looks like your angelfish might have tapeworms, you should look into medications like Fritz Paracleanse, Aquarium SolutionsPrazipro or API General Cure, which will all deworm your fish. Look for online pictures of tapeworms on aquarium fish to see if they look like your Angel's poop.

 

Hope this helps!

[Discus and Rainbowfish together? Temperature?]

What do you all think? Is it possible to keep them together, and what would be the ideal temp for them? I have heard that food competition might be a problem, but if I pay attention to that, I should be okay, right?.