The box houses 4 AP300 air pumps. Each electrical receptacle has GFCI outlets connected in parallel instead of serial to make it easier to isolate and troubleshoot potential electrical problems. Also, I can use the GFCI Test button as an on/off switch for the water pump. This tank has a hinged cover and a stub out for the second tank. Used Uniseals on the tanks and bulkheads on the troughs.