I just bought the 500 from my LFS today. My Max-S has the original sump. The Red Sea site says to add the ReefMat to the left chamber (the largest section). it would hang on the very front of the sump. I have a dual reactor sitting in that right now but planned on removing it anyway. I will run the media a different way now. My LFS advised me to actually place this in the back right section. This is where the overflow tubes currently run down to. They told me to remove the small glass panel between that small chamber and where the filter socks are. Doing this would still open up that left compartment for additional equipment, etc. Here is the diagram from the RedSea site with their recommended placement. If you have added one of these what did you do? I like the idea of using all the spaces/sections if I can.