If you use a false bottom, the undergravel feature is not needed but a pump to move the water is. The water is cleaned as it drains through the grave or substrate as it drips or flows back down to the false bottom.

You put in the false bottom, lay the substrate screen over that then place your substrate or stone or what ever on top of that.
Have a location that you can access the water with a hose to drain it on occasion for changing or in case of some unknown future problem with the water.
What ever holds the false bottom up should also have drain holes through it so there is no way to have standing water that you can not access or have moving.

Hope this helps.