Heater hasn't been working and now he's very inactive

[Irishdancer]

The last two weeks or so, I've been having trouble keeping Anakin's water warm enough because his heater isn't very strong. I've not seen much of him except during the night, because I've been working long hours, but until today he was eating normally and he'd respond if I touched the side of his tank, so I thought he was doing okay. He's never been very active during the night, and I didn't see how he was acting during the day. But today I've been home, and he's barely moved at all.

Housing
What size is your tank? 2 gallons
What temperature is your tank? Usually around 80, more like low to mid-70s the last two weeks
Does your tank have a filter? Yes but I only use it occasionally
Does your tank have an air stone or other type of aeration? No
Is your tank heated? Yes, but the heater doesn't seem to be working very well
What tank mates does your betta fish live with? None

Food
What type of food do you feed your betta fish? I alternate between soaked pellets and a high-protein powder for fry
How often do you feed your betta fish? Once a day

Maintenance
How often do you perform a water change? Usually twice a week, but I missed the last change
What percentage of the water do you change when you perform a water change? About 50% once a week and 100% once a week
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?

No, I'd just changed the water earlier today right before I noticed the problems

Symptoms and Treatment
How has your betta fish's appearance changed? Not at all
How has your betta fish's behavior changed? Very inactive. It might be my imagination, but he also seems to be breathing more heavily
When did you start noticing the symptoms? Today, but it's probably been longer
Have you started treating your fish? If so, how? No
Does your fish have any history of being ill? No
How old is your fish (approximately)? I've had him about three months, and he was probably less than a month when I bought him, so maybe four months?

[sainthogan]

I would start by getting a new heater. Temperature fluctuations can be extremely stressful and cold water can cause the symptoms you are describing.

[AyalaCookiejar]

Yeah, this is probably due to a cold temp and missing a water change... He's probably stressed from being cold and being exposed to ammonia.

Why do you turn the filter on and off?

[Irishdancer]

I used to use the filters all the time when I was having troubles with protein film, and some of my fish didn't like them because they made a bit of a current. So I got in the habit of only using them when I was in the room, and even less often now since the tanks don't get protein film anymore. I'm especially wary of using his when I'm not there, since I bought him when he was very small and I was afraid it'd suck him in.

I've switched his heater with another fish's for now. I've been looking for another heater for a while and not been able to find a good one for his size tank that's in my budget, but this one will at least keep the temperature stable. The water is currently at 78 degrees.

[AyalaCookiejar]

Turning the filter on and off is worse than not using it at all. Either you turn it on and leave it on, or don't use it at all... Protein film isn't anything to worry about, anyways.

I'm glad you found something that's working for now.

[Irishdancer]

Today he's the same. He'll react to me if I disturb the water, but otherwise he isn't moving at all. He just now came to the surface for air, and he didn't seem to have troubles swimming, but then he just sank to the bottom again.

[blu the betta]

when the wters cold their matabolism slows down so i think thats whats happening.