For 50 year, fishkeeping enthusiasts have looked to the Tetra name for products and solutions that add ease and beauty to their home. Whether you're an experienced hobbyist or just starting out, Tetra has everything you need–from a large variety of quality fish food and innovative equipment to test kits and decor. Algae growth can quickly take over a pond and cloud your water. Conquer algae growth with TetraPond products so you can keep a beautiful oasis. TetraPond Pond Block removes pond algae for clean and clear pond water. Pond Block helps control algae growth in ornamental fish ponds. Use one block for water volumes between 100 and 250 gallons. Replace block when the previous block has fully dissolved for regular maintenance. Dissolving usually takes about 30 days. This pack contains 4 blocks. TetraPond Pond Block is for containerized ponds and fountains only. Not recommended for use with live plants.