Fish has Ich! please help!

Ok so my boyfriend wanted to get a fishtank so I bought one and we got 2 medium sized goldfish. What started small turned into a big hobby. Now we have 1 small goldfish, 2 medium sized goldfish, 2 fantailed goldfish, 2 koi, and one small plecos. I also bought a 50 gallon tank for now, I know I will have to get rid of some now, and I now also know that all these fish don't go together, no thanks to the pet store people. Anyways, we bought the koi from Wal-Mart.... big mistake.... they had ich! I had no idea because I am new at this! We had them all in the little tank for a couple of days and have had them all in the big tank for a week now. So both tanks have been infected with this parasite. And last night I spotted one dot on one of the medium sized goldfish. I think I should go ahead and treat the whole big tank because it's all infected. I was thinking about starting with the salt/heat treatment and if that doesn't work, then go to rid-ich or something like that. About half the people I have talked to said plecos aren't good with salt and the other half said they have treated with salt and their plecos are fine. So I am just wondering if anyone has any input on this treatment. Also, I am wondering what is the quickest way to get rid of the ich in the tank with no fish? Please help, any information would be appreciated!​