[helene schoubye]

But I think that theres maybe not a ‘general rule’ here. I have normally only small tanks of 12 or 20 liters, and I never use either pumps, nor oxygen bubbels or anything really, and I never have problems with bad water conditions or too much build up of waste.
I wanted to write, that I also never have more than a pair in one tank, and that is actually mostly true, – and of course this then means that theres no way two small fish fed on live food is polluting the tank, – at least I have never experienced it.
But then I remember that actually sometimes I have a breeding tank of 12 liters, where a lot of fry survives, and since they are impossible to get out, I sometimes have 10 paros in 12 liter in different sizes from adults to subadult to small small :)..
I might be more aware of changing water for a while, but I do not have filters, and do not get any problems with too much bacteria or stuff on the surface.
So I dont – at least – see your experience with your 5 gallon as any kind of ‘proof’ that water becomes stagnant if you dont circulate it.
I dont say its a bad idea though, if you can find a small bubble filter which can move slow enough to not whirl the whole tank around 🙂 .. I think paros are used to a slow stream, and thats its probably good, – but in very small tanks, with few fish, it can work well.