Santa Monica Filtration HOG2 Upflow Algae Scrubber with Green-Grabber Textures, 2 Cubes Feeding per Day Great for freshwater, removes ammonia, nitrite, nitrate, phosphate, nuisance algae, CO2 Can't overflow onto your floor, because it is always under water You can feed some of the algae to your fish or snails No electrical parts go in the water; only air Can go on aquarium or sump wall if 5/8" (16mm) thick or less