Hi everybody, this is my first post, so please bare with me. I've had bettas for years and never really had big problems. I just recently lost one of my oldest babies

, I think he was close to 5 years old and I think he had farely good life.
I was wondering if someone could help me with my recent problems. My Snowflake stays at the bottom of the tank, kinda lays on the side most of the time. I used to have him in 3L condominium, but I guess water temperature was going up and down too much so his imune system got weak and he has fin rot and later he probably got SBD. I was using salt and couple of drops of aquarisol for a while everytime I made the water changes. I was always doing regular 100% water changes and I never had problems with my other bettas. I moved Snowy almost 2 weeks ago to 5gal heated tank and I keep temperature around 78. He is not constipated, his tummy looks normal, I gave him peas anyway to make it moving, just in case and he loved it as a treat. He has apetite an I keep feeding him little bit at the time and I mix pellets, flakes, shrimp. He is relatively happy fish, swims up for air, comes to say hi, still eats fine but can not swim like "old himself". I went to a pet store yesterday and they recommended Maracyn Plus. I gave him first dose yesterday. His fins are almost looking that they are growing back (even before I start medicated him) but he still lays on the bottom or on the plant leaves. I have a filter going but no carbon because of the medication (or should I add it and medicate with carbon in the filter?) I 've been checking the water and reading always is:
Nitrate 0
Nitrite 0
Total hardness 0
total chlorine 0
total alkalinity 0
PH 6.8
Is there any way I can make him feel better and swim normal again? Thanx for help