Please help. I am unsure of the next step.
My long finned betta recently has lost a significant amount of tail in a month. I can not tell if it's fin rot or tail biting or something else. 
My halfmoon betta has a 5 Gallon tank with a heater sponge filter and some live planta he is currently in the cube 1.5gal hospital tank with a heater small sponge filter and a silk plant. He has been in there for a month about and he was put in there originally because he was not eating and unable to come up for air.  I saw his fins were getting torn but I wasn't sure if it could be fixed but I have put an almond leaf in his hospital tank to see if it would help but it's been about two days and he hasn't changed much. He has been eating 1 to 2 pellets a day and he has been interacting with us when we come up to feed him even swimming around looking at stuff and the tank beside him. His hospital tank has ph of 7.4, ammonia levels of 0.25 and nitrites are at 1.0. I did a 25 percent change in his hospital tank and have been daily. The water is currently lowered because he was struggling to swim. He lays at the bottom a lot and rests a lot but he always lays on the bottom with his head up and body on its side and every time he stops swimming his body falls to the side so he uses the tank wall and the silk plant to keep upright.
I have Kanaplex but I have NOT used it yet because I dont want to harm him. if i use the kanaplex how would I do it and what should I do?

Please Help Sick Betta