Need help fast, Stiff platy

My wife just noticed one of our male platy's floating around in the current of our 55 gallon community tank. He is still normal sized, no external damage but his body is bowed and stiff. He can't control his tail but his fins are all moving and his gills work. I put him in my hospital tank but I don't know what to try and treat him with. Anyone know what type of problem this is? can it spread and if so, how can I treat my tank to keep the other fish from catching it. None of the other platies or mollies seem to have any problems. Kyle

I went ahead and flushed this platy. I turned all pumps/filters off in the hospital tank and he couldn't even swim at all. No way he'd have made it. However, on closer inspection, I found the female that we got at the same time with him (probably 6 months ago) is MIA and another male platy doesn't seem to have 100% control of himself. Two platy's swim side by side, one is perfectly still and the other looks drunk. any ideas? I did notice one of my rummynose tetras has a spot of ich or two (again) and I added Ich Guard II to the tank. I can't seem to keep my rummynose's clean. They seem to be very prone to ich. The platy's show no signs of ich but could it be something similar causing both? I do weekly water changes and my water tests passed with flying colors yesterday. 0 ammonia, 0 nitrite, 10 nitrate. My PH is around 7.8 and my water's a little hard but it's been this way forever so nothing has changed there. I have lots of tetras including neons which I would assume would show signs of stress before platy's if there was something else in the water. thanks, KYle

you lost me, I bought these in the store. The one that I flushed last night and the one that is missing were bought at least 6 months ago and the one that's swimming wierd was bought before that. How does inbreeding fit into this? Kyle

I see what you mean. Are there any treatable diseases with these symptoms though? Kyle