Specifications:Net weight: 35 tablets per bottleShelf life: 3 years Usage Mthed: 1. First calculate the volume of water in the fish tank. How to calculate the volume ?Square fish tank: V (L) = length (CM) x width (CM) x water depth (CM) (measure the length / width / water depth of your fish tank)Cylindrical fish tank: V (L) = 3.14 x cylinder radius (cm) x cylinder radius (cm) x water depth (cm) (measure the cylinder radius / water depth of your fish tank) 2. 2 Water Purification Tablets for 10L water, 4 Water Purification Tablets for 20L water , 6 Water Purification Tablets for 40L water etcif the volume of water is 10L, then put 2 Water Purification Tablets to your fish tank, if 20L water then put 4 tablets3. After putting the water purification tablets, waitting for 3days,the water will be clear, if not , then put the water purification tablets again. Package includes:1/2/3bottles *Water Purification Tablets for Fish Tank