Nano predator

[ExistingEasy]

So I’ve had a 20G tall set up for ages now mostly as a grow out for guppie fry. I want to make it into a nano predator tank... so to speak. So ideally I’d like a fish that stays roughly the size where it’s ok in the 20gallon but worst case an upgrade down the road to maybe a 40 depending on group sizing (depending on fish). But yeah every bit of research I’ve done is winding me towards the wrong direction I think. Any advice on nano predator fish or just solid bank tank ideas would be much appreciated