Meemmoo - if your going to have a DSB, your LR will start to settle if you have fish or other burrowing animals in the tank. But you don't have to put them directly on the bottom. I use 3 inch long sections of 1 inch PVC to creat pylons and set the LR on top (like 2or 3 per rock on the bottom will work). It keeps them from settling. Before I used them, they would dig, the LR would sink, they would dig more, etc. Using the PVC, they dig and then stop as the LR stops. Works great.