Lethargic Betta?

[AngesRadieux]

Housing
What size is your tank? 2.5 gallon
What temperature is your tank? ~75
Does your tank have a filter? No. It used to, but it seemed to be bother Regulus so I took it out.
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? Aqueon Betta Pellets, Hikari FD Bloodworms, Hikari FD Krill (crushed), Nutrafin Flakes (Obviously not all at once)
How often do you feed your betta fish? Two/three pellets and a small pinch of crushed flakes twice a day or 2-3 FD blood worms at night instead of pellets or a small pinch of crushed FD krill at night instead of the regular pellets. I like giving them a bit of a variety

Maintenance
How often do you perform a water change? Roughly every 3 days
What percentage of the water do you change when you perform a water change? 100% It may seem a little excessive, but his water's been getting dirty very quickly, probably because some of the flakes are sinking when he doesn't eat them.
What type of additives do you add to the water when you perform a water change? I just switched to API stress coat conditioner. I used to use Nutrafin Aqua + conditioner

Symptoms and Treatment
How has your betta fish's appearance changed? He looks a little thin to me. Some of the scales on his head seem to have lost some of their color, as well. Though, there hasn't really been a significant change in appearance.
How has your betta fish's behavior changed? It's not much of a change, but he seems more lethargic than usual. He's spent almost the entire day resting on one of the leaves of his fake plant that's pretty close to the surface. He's not really eating much. In fact, I put two blood worms right in front of him and he just ignored them. One of them I wiggled around in the water to try to entice him but he still ignored it. He's not usually an especially active fish, but he seems especially sluggish today.
When did you start noticing the symptoms? After his water change today
Have you started treating your fish? If so, how? I haven't really done anything other than try to get him to eat some food by placing FD bloodworms and pellets right in front of him.
Does your fish have any history of being ill? No, just unusually shy
How old is your fish (approximately)? I've had him for about 2 weeks and he was in a new shipment of fish that had arrived at the pet store the night before I bought him, so I'd guess not too old. I'm not sure what age they are when the pet store stocks them, though.

It may be absolutely nothing, but I'm a little paranoid.

[Sakura8]

You could try bumping up the heat. 75 is on the low side for their temperature range. Try to get it up to 78-80 degrees.

[AngesRadieux]

Sadly, Regulus died last night. Though, I'm not sure if it as because he was sick or if his fin got caught under one of his caves, since he as at the bottom of the tank when he died and one of his pectoral fins seemed to be damaged. :/ But I didn't just put the cave in yesterday... There was never a problem with it before... Maybe it was because it as in a different place since I rearrange his tank every time I change the water? I'm just confused.

[Sakura8]

I'm so sorry! :( There's a distinct possibility that since you've only had him a few weeks that what killed him is something internal that he had at the pet store. I sincerely doubt it's anything you did.

[AngesRadieux]

Maybe... He just seemed so healthy in the pet store. He was part of a new shipment, so he hadn't been in the cup for long. His water was clean and he was alert and responsive to people. It's comforting to know that it's probably not my fault, though. Maybe I'll look into getting a new one... I plan to go to Petsmart today, anyway. Last time I was there they had a lovely blue and white butterfly. But I'm a little wary of blue fish right now. Knowing my luck, it'd come down with the mystery illness that seems to only affect blue fish.

[Sakura8]

I can't imagine how it could be your fault. Judging from the info you posted, you take great care of your bettas. Poor guy must have been sick all along. At least with you he got to pass in clean water surrounded by love.