get the tang out of there ASAP and return it to the store. when they give you store credit, YOU CAN LEAVE the store and COME BACK LATER WITHOUT BUYING ANYTHING NOW. then watch your fish to see if they get ick. if they do its all over your main tank and if you have any LR / LS, inverts etc you cant perform hyposalinization and cant add copper, so then your only choice is to A) hope it goes away by itself, not likely , but possible. or B) medicate your main tank with with any addittive you have, and prepare to do water change about 50% after medication. i have used kick ich before cause i have been in a similar situation with the ick in the main tank, and it does save lives, especially if treated early, and correctly. only those fish with the most severe cases wont survive. but know that any medication will add further stress to the inhabitants. at this point you should be realizing that your life would have been a whole lot easier if you had a hospital tank already set up and cycled and running for just the situation you are experiencing. it would also have come in handy for quaranting any new fish for 1 month before adding to main display. i suggest start with a 10- 15 g. its easy and cheap to set up.
