New Petco Betta not eating, looks sick, swims funny, lays at bottom of tank...

Problem: I got a half moon Betta a week ago from Petco and he still hasn't eaten! He sits in one corner of the tank and hardly moves. I caught the fish laying on it's side for awhile at the bottom of the tank and I checked and he was still alive. I saw something red floating in the water and I believe it to be part of his fin. He did have a long clear string hanging from his body so I think it is poop, but he still looks pretty bad. The tank is brand new: 3 gallons, with heater at 80 degrees, a filter, bubbling system. I first gave him pellets, then switched to flakes and some bloodworms. But I eventually fish them out at the end of the day. Petco has a 30 day guaranatee on fish and I needed a fish for my animal behavior class (I need to teach him a basic trick within a month). Any suggestions, or should I exchange him for a healthier one? He was 14.99... Any suggestions? I'm pretty sure I'm just gonna return him for another one at this point. I'm fairly certain he came with a disease.






Housing
What size is your tank? 3 gallon
What temperature is your tank? 78
Does your tank have a filter? Yes
Does your tank have an air stone or other type of aeration? Yes
Is your tank heated? yes
What tank mates does your betta fish live with? none currently, (may get a frog eventually, none of the pet stores have them here)

Food
What type of food do you feed your betta fish? He spat out pellets, then have been giving him flakes, but he hasn't ate any, I fish them out at the end of the day. Tried bloodworms too
How often do you feed your betta fish? twice a day

Maintenance
How often do you perform a water change? twice a week
What percentage of the water do you change when you perform a water change? partial, so maybe 50%
What type of additives do you add to the water when you perform a water change? dechlorinator drops

Water Parameters: havent tested it, but I've been using betta water for now, (or at least first few weeks)


Symptoms and Treatment
How has your betta fish's appearance changed? Doesn't appear bloated, (so I don't think it's constipation)
How has your betta fish's behavior changed? He sits in corner at bottom of tank not moving, swims a little crooked when he does, doesn't eat, but does come up for air once in awhile.
When did you start noticing the symptoms? When I got him on Tuesday
Have you started treating your fish? If so, how? I put a heater in the tank, gave various foods
Does your fish have any history of being ill?
How old is your fish (approximately)? No idea

The photo where he is flaring his fins and colorful was last Tuesday. The ones where he is laying at the bottom on the rocks was tonight!