This is called a "clarifier tank". There are two of these tanks, each of which receive the waste-water from 2 of the 4 fish tanks. Water enters through the two pipes at the near side of the photo, then has to pass downward to get around the blue plastic baffle that's in the middle of the tank. The heavier solids (most of them anyway) continue downward and settle at the bottom of this tank, where there is a drain where the solids can be flushed out. The water without the solids then comes back up the other side of the baffle and exits through the single pipe at the far side of the tank. There's a screen covering this end of that pipe to catch any solids that make it past the settling.