Betta's SBD issues came back

Housing:
How many gallons is your tank? 10
Does it have a filter? Yes, HOB
Does it have a heater? Yes, 2 50w
What temperature is your tank? 76-78F
Does your tank have an air stone or other type of aeration? No
Does your Betta have tank mates? None.

Food:
What food brand do you use? Hikario Bio-gold / frozen brine shrimp
Do you feed flakes or pellets? Pellets
Freeze-dried? No, never
How often do you feed your Betta? How much? about 10-12 pellets a day in 2 servings

Maintenance:
Before your Betta became ill how often did you perform a water change? About once a week
What percentage of water did you change? 30%
What is the source of your water? Tap water
Do you vacuum the substrate or just dip out water? Vacuum
What additives do you use other than conditioner? What brand of conditioner? Seachem Prime

Water Parameters:
What are your water parameters? Please give exact numbers. If tested by pet store please get exact numbers. "Fine" or "Safe" won't help us help you. Important: Test your water before the regular water change; not after one.

Ammonia: 0 ppm
Nitrite: haven't tested
Nitrate: haven't tested
pH: haven't tested
Hardness (GH): Don't know
Alkalinity (KH): Don't know

Symptoms and Treatment:
Haven't done much for treatment yet, besides increasing his diet more biased towards brine shrimp instead of the pellets to give him as many nutrients as possible. I have kanaplex, aquarium salt, and epsom salt.

So when I first got my betta from the pet store several months ago, maybe 3-4, he had SBD issues. As in, his butt would sink faster than the rest of his body. It was noticeable, but he could swim fine and his behavior was normal. Since getting a new heater, he recovered and for the past months he has been doing just fine. His butt would still sink, but to a much less degree and it didn't impact his swimming like it did before slightly. Behavior normal, fins and body are in good shape, eating normally, active and flaring when I use the mirror. As of now: his fins are in good shape, he is active, he eats fine, and his body/scales are fine, no weird markings or bloating or anything. My tank is cycled, but I haven't tested the water. I will do so soon with my master test kit and update this thread with the readings.

However, starting this morning, his previous SBD symptoms have suddenly come back, and maybe gotten a little worse. Now, if he isn't actively swimming he will sink to the bottom butt first. If he is peddling at the surface, he will sink butt first so he is pointing up. Additionally, he will swim around normally but in order to stay upright he has to swim harder, and after awhile of this he will get tired out, and sit on the substrate tilted over. After about 30 seconds or so resting, he will get up and swim around. He will rest in the leaves and stuff in my tank, but too much swimming will tire him out. Additionally, he might be going up for air slightly more often, but it's hard to tell. I will upload a pic of him on the substrate and a video of him swimming.

I have a strong feeling this is related to my temperature struggles with the tank (it's been somewhat cold here in winter (by cold I mean 72 on my floor and mid 70's in my room)), but of course any other ideas are appreciate. Can fish just naturally struggle with SBD or have conditions genetically? He's had a problem sinking butt first since the first day I've had him. I'm not too worried about him, but I do want to catch anything bad as soon as possible, because otherwise he is healthy and seemingly doing fine.

edit: I keep catching him lying on the substrate. I don't know why, he has plenty of leaves to rest on instead of the substrate. He isn't gasping and he doesn't spend much time on the substrate, it just confuses me.

video: IMG_2756.MOV