Just a comment Cartman...there are many legitimate reasons for quitting the hobby.....money issues, time issues, moving, etc.
You are young...quitting is an easy habbit to fall-in but difficult to change. There is nothing wrong with making mistakes in this hobby....we all have
I really believe you did nothing wrong...the animal was sick or in a weakened state when you purchased it. . Follow the advice I gave you in the other thread regarding selection of animals for your system.
He may pull through.....I have had fish that I thought would NEVER make it and yet they did. If all your water parameters are in check there is really very little you can do....accept possibly moving him to a QT tank ...but he may not survive the move.
Since you did not have him in QT.....leave him be and perhaps tomorrow he will pull through for you.
I would return him should he not make it...but the lfs may not warranty their fish. In which case you are at the mercy of the manager and/or owner as to if they will make an exception