New fish, sbd?

[Oiseau]

I've had my new betta for about a week now and I'm a little worried about him. He wouldn't eat for the first few days, but I figured he was just adjusting to the new tank etc, then ate for two days, and now has gone back to not eating. Then, when I came home today, his backside was drooping quite low. He wasn't having troubles swimming (only drooping when still), but I did a 100% water change and cleaned out all his ornaments, etc. Now the drooping backside has gone. :/

Housing
What size is your tank? Approx 3g
What temperature is your tank? 76
Does your tank have a filter? No
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? Pellets
How often do you feed your betta fish? 1 pellet, 2 - 3 times a day

Maintenance
How often do you perform a water change? 3 times/week
What percentage of the water do you change when you perform a water change?One 100% and two 25% weekly.
What type of additives do you add to the water when you perform a water change? Water conditioner

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

Ammonia:
Nitrite:
Nitrate:
pH: 7.0
Hardness:
Alkalinity:

Symptoms and Treatment
How has your betta fish's appearance changed? His tail was drooping lower than his front, but it's not happening any more.
How has your betta fish's behavior changed? Not eating.
When did you start noticing the symptoms? Not eating - a couple days ago. Drooping tail - about an hour ago.
Have you started treating your fish? If so, how? 100% water change with conditioner.
Does your fish have any history of being ill? Not since I got him.
How old is your fish (approximately)? No idea. I've had him a week.

[Oiseau]

Update: He showed some interest in his food today, grabbed it and then spat it back out, but didn't eat.

Anyone have any ideas? :/

[inkrealm]

have you tried changing his food? sometimes bettas act spoiled and start rejecting food if you give it to them on a regular basis.... I can't see any issues with your info... maybe I missed something though.... technically you could do one 100 and one 50, but I think it'd still be okay with two 25's and one 100....

[Oiseau]

I've tried feeding him flakes too, but he has zero interest in those too.