Sick Platy

Hey all,
On sunday I added 4 red platys to my 20g. Unfortunately while my dad was netting them out of the bag one fish leaped like a foot in the air and landed on the tile floor! I dropped on my knees and he flopped into my cupped hands (i was worried about handling him) and i put him in the tank. Later, after careful inspection I found all the fish to be free of ich, three were healthy,colourful and with well formed fins, but one, probably the jumper) seemed to have some scale damage. Ive carefully examined each fish everyday since then, they others are adujusting nicely but the one is off colour, constantly hiding, not eating and having trouble swimming-it seems his bottom fins are paralysed or damaged and he wont use them. He also attacks any fish that comes near him-and i noticed a white spot at the base of his tail-at first i thought it was ich but it looked kinda like a missing scale-anyway its faded or gone today, and so far no signs of ich any place on him or the others. Does anyone know what this mite be? is it contagious? im worried that hes suffering but im reluctant to "euthanize" him.
Thanks

Surprisingly the other three fish are very peaceful and "kind" to the sickly one-its cute. The sickly platy will hide among the fake plants, sometimes rising 3 or so inches from the bottom but very rarely-he cant swim well without the use of his bottom fins and he has to jerk around-I feel really sorry for him. I dont think hes been eating because he never swims to the top for the flakes unless (hopefully) he could get at the odd fpiece of flake that sinks to the bottom. So far no signs of any ich on the fish-I just did a 30% water change and the water quality is good. i'l definetly try to get that Melafix stuff if you say it works-I found out too late about Cycle and Waste COntrol...
Thanks