My sunshine peacock became aggresive these past two days and I had a 35gal hex tank broke down and for sale so I put it back in service . I used 20gal from my 75gal the fish lived in anyways and 15gal of fresh water I thank this will be safe for the fish and help with cycle. How long should I keep him sepprated befor I reinterduce back to 75gal I plan to rase fry in there when he is back in other tank or sold I like him one of my fav. but he cant hurt other fish "bad fish" so what do yall thank.

Water holds little if any beneficial bacteria. Gravel and filter hold the most.

The tank will not start or complete its cycle without you adding a food source to get the bacteria multiplying :)

You should be okay on the cycle, but i'd be testing the water just to make sure you transferred enough of the established filter media to keep up with the fish. As for selling him or give him another go in the tank, i'm not sure how to advise you, but he is a pretty little jail bird. I can see why you are hesitant to give him up.