Tetra Pond Sludge Reducer BlockTetra Pond Pond Sludge Reducer with Clear Zyme Technology blocks are bio active cleaning blocks that will keep water clean and clear by breaking down wastes which accumulate on the bottom of ponds as a sludge layer. Keeps pond water clean and clear Bio active cleaning block for pond maintenance Breaks down wastes which accumulate on the bottom of ponds Also great to use during the setup of a new pond and the spring start up or fall shut down of