The sandbed is 4-8" higher in the middle and goes down as you get to the sides because the overflows have holes at the bottom. The device in the corner is a Tunze Stream Pump, this is a great pump for current, I have one in each corner that come on opposite each other. These two pump replaced 4 Maxijet 1200s and an ampmaster 3000. Each pump puts out about 3100GPH and you can control it and turn it down to 30% and have it do alot of other things with the controller unit.