The central trough, where the biofilter pipe will lie , is lined 'in case of overflows or blockages in the pipe. If the pipe overflows or breaks, the water will simply flow back into the fish tank. Although harder to make and use, 'pipe-clips' will last longer because they won't rust. Placing the biofilter pipe. Carefully lay the biofilter pipe into the central trough, being careful not to tear the plastic or break the irrigation bar.
Water flows through the biofilter pipe because the entrance to the pipe is higher than the exit.
Ideally, the biofilter should be mostly full of water, with a thin layer of air at the top. Setting up the air-lift pump. You will probably have to widen the hole a little bit with a knife. Cut two 5' long pieces of air tubing and hook them up to the outlets of the air pump. Attach an air stone to one of them.