We do not have the flat tank version. Which filter did you install? We have the quite fine one with a rinsing function. Thus, very little contamination in the tank. Our pump also tolerates dirtier water. But there is not much in it when I look at my filter after the pump. With the right filter on the inlet, you almost never have to clean the cistern. Maybe it works with a pool robot with a coarse filter? Otherwise, a small compressed air bottle and pond vacuum. I wouldn't do it without aeration either.