I've tried having a 1 fish aquarium before and I did really enjoy having a large dog like pet fish (8" oscar in a 55g by himself) after time it just came down to wanting to have a variety of species. The only fish I'd consider breaking my rule not to do that again is if I could keep a dovii for life in this 125 and after watching a million youtube videos I know that a 125 couldn't be a forever home for that glorious fishunless the other guys are enthusiasts as well i would consider a wet pet type of a fish that can handle some stress from other people