Betta Fish: White lump on betta

Hi all. My betta leviathan has recently formed a small white lump on his head. I've only noticed it today. He seems to be swimming and eating fine and not acting lethargic or anything. However he does have a small piece of his fin missing. I thought this was from a sharp piece of driftwood in his tank that ive already sanded down. I got him second hand about four months ago. He's in a heavily planted 25 litre tank thats been established for about six months. He's in with two kuhli loaches (i know they need more space, im going to upgrade him to a bigger tank) and nothing else. I already dosed him with a tiny bit of bactonex, but any help is greatly appreciated!! To me it looks somewhat like a tumor or maybe a fungal infection? The ph of his tank is 6.5, theres a very very small amount of ammonia, id say about 0.5ppm, 0 nitrites and 0 nitrates. I feed him live mosquito larvae, frozen daphnia , frozen brine shrimp and occasionally frozen blood worms. he eats once a day and i fast him on wednesdays and fridays. I feed him about one squirt of a little aerator, about 5ml.