Sorry, worded that funny. I thought it was bacterial. I was just mentioning ammonia because that's usually the biggest worry and the #1 killer with QTs.
Hard to tell with the dead fish but I could see it being bacterial. If so, I'd recommend using a different medication than Melafix. Melafix is just an antiseptic if I remember correctly.
I would use an antibiotic if possible. I usually lean on Furan 2, erythromycin, tetracycline, or kanaplex/metroplex to treat bacterial infections. The problem is figuring out if you are fighting a gram positive or gram negative bacteria so if one doesn't start working, I usually switch to another.
For my emperor, he kept getting bacterial infections in QT and couldn't shake it so I used kanaplex, furan 2, and tetracycline combined and he kicked the infection. I would not usually recommend running multiple antibiotics unless you were unsuccessful in your initial treatments. Its more of a last ditch effort, mainly because adding each medication can lower the oxygen level in the tank to dangerous levels.