Comline kits require an 11" water depth minimum. You need one mechanical filter and one Biohydro reactor per 100gals. For larger systems you may wish to use 3167, a dual mechanical filter, the biohydroreactors can be chained in series off of this filter. Each module is roughly 4.75" square. The system set 12 is 17.25" long and is submerged roughly 4". System sets require mounting on fiberglass rails, comline kits simply clip together. System bio reactors are each suitable for 50gals. In both cases coupler 3178.71 is used to connect reactors to mechanical filters. The mechanical filters serve as the carbon source for the denitrating bacteria. The mechanical filter cartridges are about 1" thick spiral wound floss. They should be rinsed or changed weekly, they wash well in the washing machine when unravelled. Their filtration is similar to diatom filters in effect.