Mbu puffer ill, plz im a noob

[Maxwe11 1993]

My mbu puffer seems to be very unwell, I may have left it to late already I'm nt sure really. He started to act strange but flying around the tank banging into the glass a few times then calming down, something obviously bugging him. B4 this he stopped eating and till this day hasn't eaten for 1week-2 weeks now. He seems to have developed white string like fungus on his tail. I have been doing regular water changes, and Tuesday had the water tested and everything seemed fine, just got told to do a water change even though I did one the previous friday. I bought "eSHa 2000" fungus treatment after showing pics to the fish specialists shall we say. I have completed the 3 day treatment but am still worried as he seems to just hover at the top of the tank for ages. The fungus like stuff has gone now but instead is left with a redish cut/ rash. I knw it takes time for medicines to work but an worried as never experienced fish illnesses b4. Sorry if this I'd common fish disease but I just dont hav the time to read threads ATM. I do apologise. Thankyou.