Nitrifying Aquarium Sand Tank

Guess some would call this an above-tank refugium. Explanation: I am working on an aquaponics system. Putting my fingerlings outside during the winter would involve a $55/month heating bill in the aquaponics tank (250 gallon), I decided to raise them indoors in a 70 gallon tank until spring. I do not have any substrate at the bottom of the tank but use 2 Aquaclear top hang-on filters and seaweed. Well, I realized that my system didn't have any anaerobic bacterial location. I liked not having the substrate at the bottom as it made for cleaning the bottom easy. Thus launched this experiment to put the substrate in a carrier on the top of the tank. The carrier is a 11.5" x 15" x 7" plastic bin from Target.

The system is very simple. I have 4" of Aragonite sand in the carrier. There is about 2 cups of coarse river stones on the top. Six 3/8" holes are drilled at the top of one side of the carrier on a downward angle. The carrier sits perfectly on the top lip of my tank (a happy accident). One of the Aquaclear units drains directly into the carrier, and the water then exits from the drain holes. Note, that the feed pipe for the Aquaclear had to be extended with 0.75" tubing to allow it to be curved around the side of the carrier. Also, there is a small length of 0.75" tubing on the input tube as it sits in the Aquaclear -- this allows me to position it when starting to prime the filter. The lid (not shown) is trimmed to allow covering the refugium to minimize water evaporation.

Pictures show the unit in operation as well as the draining action.