Haha, they're good beggers indeed.
I think the water stays so clean because the filter(s) have a lot of overcapacity.
And i think not feeding to much also helps, my turtles grew way to fast when i fed them a lot, so know I feed a lot less, they get a little piece of waterplant every day and different pellets every other day. And I don't feed the guppies and the shrimps, they'll eat whatever the turtles don't, they even eat what the turtles díd eat, if you know what I mean

I only change the water once every 1,5 to 2 weeks and that's not because it's dirty or smelly, but i think it is just "fresher".
My tank is in the livingroom, in between our couches so it wouldn't be good if it would get smelly, but luckily that never happens

I do use a spong every week to clean the alge of the glass and decorations. Then afterwards it's a little cloudy, but the filter always takes care of that in 2 ours or so.