Is this used to filter the water? Do I need to buy a filter for it?

This kit comes with a sponge filter. No other filter is required to keep small bits of grime out of the pump in a small pot water feature. Of course, you will need to lift and clean off the unit and flush your water feature occasionally. How often will depend on how much stuff is falling into your little fountain. It's not going to filter it like drinking water.

it doesnt filter water, just turns it and airrates a little., it has a sponge filter