The basis for your clean pool
The filter and pool pump are the basis for obtaining clean filtered water. 
To obtain and keep the water clear and clean it's important that the pool water is pumped through a (sand) filter.
If you would not filter the water, bacteria, algae and fungi get a chance to develop in the pool water and adding products to your pool water is not very useful.

How much flow rate should my filter or pump have?
To know how much flow rate your swimming pool filter and pump must have, there is a general rule: volume of the swimming pool in cubic metres (m³) divided by four.
So: number m³ : 4 = the necessary flow.  
E.g: 18 m³ : 4 = 4.5 m³/h for this you have to take a pump and/or filter that is at least as heavy, certainly not lower! It's always better to have some margin than to calculate with a small margin. 
You can be sure that the water is sufficiently filtered and the more the water is filtered, the better the quality of the pool water.