HBH Balance Blocks 15 pack. Each block will handle a 50 gallon aquarium. Balance Blocks are a unique aquarium additive that replenishes calcium and a total of 71 trace minerals to salt water and reef aquariums. As Balance Blocks slowly dissolve on demand, vital elements are released into the aquarium and made bio-available for use by fish and the vast number of organisms present in a reef aquarium. Balance Blocks will help stabilize pH at an ocean level of 8.2 - 8.3. Tests have shown aquariums with the addition of Balance Blocks have improved development, health, and vitality. No marine aquarium should be without the micro-nutritive and replenishing effects of Balance Blocks.