I am not really sure. I bought this tank uncompleted, but with substrate from someone near me.
It looks like stones, and I think this may have to do with the fact that the waterfall pump is huge, and spreads out almost over the whole tank, so I feel like it may have a certain water capacity that it is needed. To back up this thought, every time I take water out of the tank from the waterfall, the pump makes a grinding sound and small bubbles come out along with the water, almost like carbonated water.
Any suggestions?
(there is not a false bottom in the tank)