Hmm.. Okay then. They're not particularly susceptible to water quality, but definitely keep up on water changes. How often do you feed? A 16g tank will not have enough pods to keep your leopard satiated for long. I would be feeding at least 3x a day with a good frozen food, such as PE mysis, or Larry's Reef Frenzy. That's your best shot. Once the leopard gets through all the pods in your tank, the health of the fish is going to drop.
Also, remember that it's in the fish store's best interest that you keep the fish, as they make money off you. People on this forum have nothing to gain from telling you to return it, and we're just looking out for the fish.
I will take your advice on food. As for the fish store I really trust them even if they gave me bad advice. They said they would happily take it back no questions, the ball was in my court to return it. So I cant blame them...I usually google everything, but when you have two kids with attention sdpans of flies sometimes you make decisions without proper research.
He's eating the flakes a little, but I've noticed too much goes wasted (which will destroy my tank). He's not the type of fish to go bananas for the fish food.