Step 3 relates to the plumbing, the rough in for the filtration has started. On one side is the intake which includes a surface skimmer, the two valves will control how much flow is skimmed from the surface vs a submerged strainer (still to be added). The skimmer box will also hold items like a float valve for level control. The output from the filter is roughed in on the other side of the tank.
