[Enkil]

Either tail biting or something in his environment.
Keep his water very clean and warm. I have a couple of tail biters and they get along just fine. Try rearranging his tank and see if that helps things.
What kinds of decorations/plants does he have?

 

[Enkil]

For an ammonia spike, I recommend just doing a partial water change as suggested. What size is his tank? Is it a live plant?

If he has no black edges to his fins, I wouldn't suspect fin rot. Long finned males can be prone to biting. There are many things behind it. If he is biting, there are quite a few things you can try to stop it.
He may be bored. Try rearranging his tank. Add more to it. Give him exorcise once a day using a mirror.