Please help me help my poor Betta!

[Kpabs]

So, a few days ago I posted that my fish, Beibs, wasn't doing too well. He seems almost lethargic, swimming at the top of the tank, or just sort of... floating there. Sometimes he's completely on his side, but when I pick up the top or make any movement around it, he almost swims frantically away. But then he pauses, and either doesn't move, or floats with his head straight and his body on his side (just now, though, he was completely on his side). This morning, he was down by the heater at the bottom of the tank, but almost floated to the top when I turned the light on. He's still eating... I feed him 2 Hikari Betta Bio-Gold pellets in the morning and 1 at night, but I've gone 24-hours earlier this week without feeding him to see if it would help at all. No luck. I'm really worried that something is going on and I don't know how to fix him! His stomach is a bit extended, is it possible that he could be constipated? There has been poo in the bottom of the tank (and I've seen him going to the bathroom), so it's still passing... but what else can I do? Please help! Could it be SBD? And if so, how can I help him?
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Housing
What size is your tank?; 2.5 gal.
What temperature is your tank?; 78-83
Does your tank have a filter?; yes
Does your tank have an air stone or other type of aeration?; no
Is your tank heated?; yes
What tank mates does your betta fish live with?; none

Food
What type of food do you feed your betta fish?; Hikari Betta Bio-Gold pellets
How often do you feed your betta fish?; Twice daily, two pellets in the morning and one at night.

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?; 50%
What type of additives do you add to the water when you perform a water change?; Prime

Water Parameters:
Have you tested your water? If so, what are the following parameters?; yes...

Ammonia: 0ppm
Nitrite: 0ppm
Nitrate:0ppm
pH: 7.4 (both in the tank and from my tap)
Hardness:
Alkalinity:

Symptoms and Treatment
How has your betta fish's appearance changed?; He doesn't seem as vibrant; stomach a little distended.
How has your betta fish's behavior changed?; Floating on his side at the surface, lethargic, being "pushed around" by the filter
When did you start noticing the symptoms?; More seriously about a week ago.
Have you started treating your fish? If so, how?; Tried fasting for 24 hours.
Does your fish have any history of being ill?; No
How old is your fish (approximately)?; I've had him for 3 months...

[Kpabs]

Here are some pictures, so you can get a sense of how he's looking... it's hard to see the floating, but I've tried to get a variety (sorry... some are kind of blurry and a little dark...)

View from above:
http://s22.photobucket.com/albums/b3...t=IMG_3995.jpg

From the side:
http://s22.photobucket.com/albums/b3...t=IMG_3998.jpg

On his side:
http://s22.photobucket.com/albums/b3...t=IMG_4006.jpg

More tilting:
http://s22.photobucket.com/albums/b3...t=IMG_4002.jpg

He looks sad. :( Help me!

http://s22.photobucket.com/albums/b3...t=IMG_3994.jpg

[Kpabs]

Anyone? He sees slightly better after fasting for 2 days, but not a whooole lot of improvement...

I also started thinking... does he have fin rot? Maybe I'm just paranoid, but the edges of his fins are kind of dark. They've always been kind of black around the rims, but if anyone else can take a look and let me know what they think, I'd really appreciate it!

[DarkMoon17]

It does look like SBD to me. I don't see any signs of fin rot, which is good.

I believe the SBD might be due to his current diet. Hikari changed their recipe so it now contains large amounts of wheats/grains so it expands when eaten and causes impaction. To treat him, use 1tsp/gal epsom salt (carried by most pharmacies including walmart). As you have already fasted him, offer him frozen daphnia to get his bowels moving.

You should consider purchasing a different pellet food that contains more real meats and fewer grains. Just look at the ingredients, you want the first few ingredients to be real meats. Omega One Betta Flakes have great ingredients, as does New Life Spectrum (for bettas) and Atison's Betta Pro.