First of all, I would have brought that fish back to the LFS immediately. At the very least he should have taken it back and given you store credit. Also, if it infected your other fish the LFS should have given you the meds for nothing or at a discounted price. If they do nothing for you then do nothing for them in the future and be sure to tell your friends how they do business. Being a owner of a LFS myself I would have taken the fish back, exchanged it and given you the meds to for free especially if it was my fault. Then again, I don't let ich infested fish out of the store to begin with. Personally I don't have an ich problem in my store since I have "Coppersafe" in all my freshwater tanks. You should have a small quarantine tank at home to use for a few days before introducing new fish to your community if you live in an area with fish like that.
To rid the ich I recommend coppersafe for you as well. Used every month it will prevent an ich outbreak from ever happening again. Some people have a problem with coppersafe for some reason, they say it's dangerous. These must be people who cannot read directions. It doesn't turn decor blue or green, you don't have to leave the light off, no need for water changes and you can continue with carbon filtration and air. Great stuff! The only exception where I would not recommend using it is if you have snails and you want them to live. Also if you have live plants or invertibrates without an exoskeleton you shouldn't use it either as it may be harmful to them. I wouldn't imagine you have snails if you bought a clown loach since it is it's favorite dish. Good luck with your fish, I hope they pull through
