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Great, innovative product. Aerobic bacteria on the outside and anaerobic in the deep core. By using in the tank or in the sump with water flowing around and not through the cubes, zero nitrates is easily achievable. Added advantage, since the material is not calcareous, there is no risk of trapping phosphates to be released later. Zero risks of "old Tank" syndrome.

I bought all MarinePure products, the Block, Spheres and Cubes over the past two years and have always been very impressed. I initially used it in a sump of a display marine tank that had very little live rock and it definitely seemed to provide the necessary biological surface for the required bacterial growth needed to keep the ammonia and nitrite levels at zero. After about a year I moved it to the sump of my frag tank. This tank only contains a few fish so there is not a large bio load but there is certainly food that goes to waste and fish excreta in the water. I’m running a filter sock and a small skimmer but no other filtration. I’ve been monitoring the nitrate and phosphate levels as I expected to see them rise but they are sitting at zero. I use a Red Sea Nitrate Pro kit and a Hanna phosphate checker. The kits are functioning correctly as I use them on my other tanks and they do show readings in these tanks. When I checked this on-line it seems that other people have had the same experience. The theory is that the outer portions of these thick blocks provide an aerobic zone where the aerobic bacteria flourish and consume ammonia and nitrites to produce nitrates but the inner portions of these large blocks are anaerobic zones that host anaerobic bacteria that actually consume the nitrates thus reducing the levels in the water. For this to work, the block needs to be in a relatively low flow area. So, in my experience, I would say that the blocks definitely help in both reducing ammonia/nitrite levels and, in some cases, nitrate levels as well.