Under each shelf layer runs a drain. This is a square PVC downpipe from Marley in which I drilled holes that match the holes in the tanks on the shelfes above. All three levels are connected on one side with T-fittings to a vertical pipe and this pipe is connected with another T-fitting to a drain that runs along the floor. In this pipe I added three openings in which I can insert a hose for when I do a manual waterchanges. At the lowest point, the drain leaves the fish room and runs into a 400ltr soak hole.
At the ceiling I have a 40mm PVC pipe running around the fish room which is connected to the water treatment tank outside. In this pipe I drilled holes and cut threads into it to fit 4mm male hose tails - one for each tank. On theses hose tails I put black plastic pipe that runs to every tank and has got a dripper. This dripper provides 8 ltr/h independent from the water pressure. This way I can controll the water that gets changed every day with a simple timer on the pump. 2x 15 minutes equals 4 ltr per tank per day.
If the pump starts, the drippers will soon start dripping water into the tanks. If the water level reaches the top of the pipe inside each tank it will overflow and into the drain underneath the tanks, from here it will flow to one end of the racks and then down into the drain on the floor. From here it flows outside and thru a filter into the soak hole.