What a horrible story!!! That is so totally not fair. If it were me. . . I would honestly wait a day or two, calm down, and write a very polite letter explaining what happened, why you feel you were treated unfairly, and explaining exactly how you are an asset to the store - include information about the tank(s) you have and what things you have purchased in the past. Mail it directly to the owner, if possible, and if not address it to management in general. Give it a week or so, and if you haven't heard back - call. E-mail is also an option. . . either way, I doubt a punk kid would bother to go to the trouble. I'm not sure if your card would have a record on it of purchases you've made with them somehow, but if so - ask them to look it up!
Either way. . . after being treated like this, I'm not so sure I'd even WANT to return! Though I would still want my money back on the card!
Good luck! I'm really infuriated for you!