How do I treat my son's goldfish with a broken tail?

I think this fish might have escaped being sucked up the filter. It is one of the really small 28 cents goldfish, have no idea exactly what kind. I have 2 other smaller ones and then 2 bigger ones about 3 inches big. The other ones are fine. I found it stuck in a plant and moved the plant a little bit so it could get out. The poor little guy sank to the bottom. I could see one of his side fins moving... but the tail and other fin weren't moving! He is still opening and closing his mouth, breathing, and looks healthy other wise. Do I put him in his own tank until he heals? How do I feed the poor little guy? Is there anything I can do to speed up his recovery? I have no idea what I am doing when it comes to fish... Also, how do I prevent them from getting sucked up again? I have a tetra filter for up to 10 gallons.... and my tank is 10 gallons... I got the smaller one because I was afraid of this! Please help!