Brush and Vacuum
A pool can still have spots behind the ladder or in corners, even when you filter it. This happens where water circulation is not sufficient, leading to the growth of bacteria and algae on the floor and wall of the pool.
That is why you need to brush it thoroughly to remove the elements and make your pool clean and safe. Swimming in such a pool can be a risk to your health. Even if you have an automatic chlorinator, never assume that your pool is clean until you brush it.