Ammonia burn or something else?

[Shallow Planet]

Tang is in cycled QT and 4 days later looks like this. Salifert ammonia test showed 0 but API showed a slight trace when compared to another QT as far as yellow vs green coloration of test.

 

[melypr1985]

This looks like a bacterial infection. A dip in acriflavin before treatment with antibiotics would help.