Oxygen Block 20g is suitable for all freshwater aquariums and ponds. The activated Oxygen Block when placed in the water will slowly dissolve and release ingredients that will help maintain a healthy environment for your fish. As the block dissolves it will replenish the oxygen supply in your tank. Replace Oxygen Block with a new one when it was dissolved. For healthy fish, change aquarium water regularly and maintain water temperature of 22-27C with a pH value of 6.8-7.2.