I think its a bit difficult, and I am not sure what is best. To know about germs etc I have too little knowledge about what is really going on, – but experience of course learns you something.
I do not change as much water as I have done in my early years as an aquarist, and sometimes I do just fill up. But then again, I have very few fish in those tanks where I would do this.
But I quite often ‘tear’ a tank down, – because with the paros I dont ‘clean’ much, – so after some months of no cleaning and plant growing, leaves deteriorating etc. – or if the tank is full of fry, – then I simply take all out and the fishes are placed in a new tank. Most of the times this ‘changing’ set of spawning behaviour, which to me signifies that the fish actually appreciate this total replacement of the water.
Another thing I quess is that once you get into the real low ph values there are probably less germs that can live there.