I stocked my tank with some guppies recently, then a few days later decided to add some more in order to hopefully stop any particular fish being bullied if any aggression broke out. Over the next couple of days, tails and fins started disappearing and fish started dying until there was only one left. I thought the reason was disease but I guess they got ill due to the stress of being harassed so much. He's been on his own over a week.
Today I added a few new male guppies and a couple of platies to my tank. All seemed fine at first, I just switched the tank lights on after an hour or so of letting them settle in, and I see my original guppy chasing half of the others, and harassing and nipping at one platy in particular. At the moment I have him isolated by trapping him in my net and just floating it at the top of the tank. He's trying pretty hard to get put and I feel quite sorry for him, but he's being way too aggressive with the new fish. The rest of the tank inhabitants are just swimming round happily now this guppy has been isolated.
What should I do? I don't want to keep him trapped in a little net forever, but I don't have another tank to put him in, and I'm not sure if I can get to a fish store to see if they'll take him off my hands.
Rearrange the tank decorations, keep him in the net for a day or two and then try him in the tank again. Possibly with everything rearranged and him last in, he'll not feel the whole tank is his territory. If he starts nipping again then returning to the shop is the other option.