I would say you could try it in a 125 gallon just make sure to keep a eye on it because you would be surpised the damage little angry cons can dish out! But i succesfully kept a jag in a one twenty five with a breeding pair but i made sure on the one end i had a cave just big enough for the cons. The combo worked out well and i ended up just moving the cons to their own tank. I would say give it a try man!