I got a new goldfish. It went through quarantine without incident and was added to the main tank on Sunday. Yesterday (so three days later) I noticed a black discoloration on one of its gill covers. Today it is still there. I think the discoloration is larger but I'm not positive.

My best guess at a diagnosis is ammonia burns. However I checked my water parameters and as usual they checked out good - 0/0/10.

Is there a delay between when a fish is exposed to ammonia and when ammonia burns appear? Is it possible that this fish either overwhelmed the filter in the QT tank or caused a mini-cycle upon being added to the main tank, but the burn is only showing up a few days after the ammonia exposure ended? Or is it something else?