Five 3 in. discus in a 15 gal. tank? They need to be in a bigger one immediately. Until then, I would be doing 50% wc twice a day. Discus may be `practically black' because of the substrate but they also might be ill. Adding more without quarantine was quite a risk, IMO. Did their colors brighten up a lot in this tank? Six is a better number than five, but not in this tank. Not much you can do about the aggression. Normally they sort it out or one has to be separated but there's no room in this tank for either to happen. Time to go shopping! Yes, there are things that are transferable to humans but I'll let someone else comment on that one. I don't worry about it. I'd get a cover, as well. Discus do jump.