I have a pugnacious male swordtail who once mated but never mated any more!( he is now less naughty )?

Question

And he even lost to a widdle female sword tail!

Answer 1

How old is he? He might be getting older and lazier, or he might be sick, or he might be needing a friendlier companion (a smaller female swordtail). Check him out to properly assess his situation and address his needs.

Answer 2

He must be really old. Otherwise such change in behaviour is not normal. Maybe you have added some other fish that have become dominant and hes not being able to come to terms with it.