They can also be used to provide current to specific areas of the tank for differing fish habits and (when dealing with deep tanks) stirring up detritus so that the filter intakes can get it. Two power heads, properly placed, can give your tank a circular current around the out edges while leaving a calm area in the center, or one directed across a ridge of rocke will not only give current loving fish like hummingbird tetras a favorite place but will leave a sheltered lee behind it for fish like neon gobies.