Male Guppy At It Again!

[gforce17]

Now that the female swordtail has gone the male guppy is pestering one of his colleague males. Is the thing to do to send this male back to the shop?

 

[snazy]

Is that a recent behaviour or the male guppy was always like that?
They are not bullies for no reason.
I know it is not a comparison, but my cat once fell from the 4th floor. He has recovered and all after a major operation, but the vet said he will suffer from arthritus eventually.
He used to be a very relaxed cat, that will never scratch or make a sound, but now from time to time he just comes and bites me. Of course, not enough to cause any harm, but I can see that he is not feeling well all the time. He always does it only to me and no-one else, which is the strange part. So they pick someone to bully when not feeling well.

Anyway, male guppies will chase each other, and it's normal as long as no nipping or damage is happening. They will also chase any female in the tank, if any, so the story with one female swordtail was also normal.
If it is an unexplained aggressive behaviour, then maybe water is not fine, or there isn't enough decoration, hiding places in the tank and it stresses out the fish. Eventually they will express the stress one way or another.

I have a swordtail female that bullied my cories for a long time after I got her, but eventually stopped. She was probably just stressed from the move. Of course there was no damage done and the cories just didn't pay any notice.