Look, here is the way to look at it. The clown will die in that tank eventually. You'll have stunted it till it eventually dies from it's inside growing too big for the outside of it's body, which will only grow so big because of the size of your tank, and then the fish will die. I don't care if the fish store doesn't take credit, that clown does not, will not and can not live in that tank. From what I've read, you need at least thirty gallons for pictus, as they get around five or six inches, and since they eat fish they're poop is just a little more potent than your average fish. Not to mention they like to have some room to swim, trust me, I have one. Take the fish back, let your tank cycle, and then we'll help you figure out what kind of fish you want that can fit in that tank without being stunted.
I don't mean to be mean, even if I come across like it, but you just can't keep fish like that in a tank that small for very long before they start to stunt.