Okay, I'll try to answer all of this to the best of my knowledge.
I'm changing the water of the main tank 50% twice a week. He has only been there for two weeks, if you checked the begging of the topic before that he was living on a 2 liter tank with weekly 100% changes.
I use tap water, which I treat with a conditioner. Yes, the tap water here is treated. The two conditioners I might use when changing the water is this http://www.labcon.com.br/peixesaquar...es/protect.htm
or this http://www.labcon.com.br/peixesaquario/condicionadores/anticlor_agua_doce.htm
The fish has been on a quarantine pet bottle inside the main tank for about 4 hours now. He has two drops of anti fungicide and a drop of anti parasite medicine inside, no salt. I also did not restricted the light on the tank yet.
The tank temperature is on 30 Celsius. The heater has little work to do, apparently, only turns on for about 5 minutes every hour or so.
He is eating well since I started heating the tank (before the heater, with the warm floating water bottles) - 5 pellets a day, all at once.
His symptoms are he is very apathic and will barely swim around at all. Before he heating started he wouldn't eat. He doesn't seem to be rubbing but the place he decided to "live on" (didn't move) was a very tight spot near the plastic decorative plant. When he DOES move elsewhere his front body seems to "drop" on the water and his back fins would go up? Like he's facing the floor. He's float like that for a while until hitting the surface, when he corrected his position. I don't see anything weird besides the yellow head. He has been apathetic for a month or so now, but the lack of appetite lasted under a week and the yellow has been there for a little over that. I might add he's been a little more attentive to knocking on the glass now that I've been heating the aquarium...
And that this has been a really warm month and my hamsters are all suffering loss of fur doing to this, so it's weird the fish is so cold!
I have him for maybe 11 months now. I cant know for sure how old he is, I bought him at a cheap fish store. Two other Betta were bought the same store the same day, they both died already. One of them died a month after the purchase of body rot and the other one died last month of hidropsy.
I'll have to mention again I'm in Brazil, so many things available were you are will not be viable here. Still I'd love any insights, thank you for looking at my case.