possible SICK albino oscar, PLEASE HELP!

about 6 weeks ago, we got a 56 gal tank and put a tiger Oscar, albino Oscar, and a 'sucker fish' . (they were all very small babies then) at first, all the fish seemed to be thriving, recently, the albino started turning black, the ends of all fins are black, and several black spots on its body. also, the albino is not getting any bigger, like the other fish are. It hangs out on the bottom of the tank, does come up for food, but then swims around almost sideways, also swimming more vertical than horizontal. My husband adores these fish, if anyone has any ideas, please share!