protruding Gill on a Betta

Question
I just bought my betta from PJ's Pets, They said he is about 9 months old.  He is a Brownish Blue color with red on his fins, the front two fins are tipped with white, he is very fat (he came fat). He eats well builds nests flares and is really active more active then my other male Betta who I have had for 6 months.  When I bought my new Betta I didnt notice untill I got him home that one gill looks like it is always flared, (when he does flare this Gill flares out enough to cover one eye).The store offered to replace him and said it may be a tumour on his stomach causeing him to look fat.  My Question is this: Does this hurt him and What is it? Will his Gill every be normal?
His tank has been ready for him for 1 month before I got him, the ph ammonia and nitrite levels are perfect for a betta, He is the only fish in his tank there is no filter or anything I change 50% of the water once every 3 days He has one fabric plant  marbles and gravel the tank is 2 liters the temp is 80.  

Answer
Hi Shannalee;

It is possible he has a tumor or something. It may be putting pressure on the gill from inside and preventing it from going in. You might try witholding regular food from him for a couple of days and feeding only cooked and peeeled green peas. Peas will help his digestion and his stomach will get smaller if it's that kind of a problem.

At Your Service;
Chris Robbins