Tetra ReptoGuard Water Conditioner Block, 12-Count (4 Packs with 3 Blocks per Pack) 12-Count (4 Packs with 3 Blocks per Pack) Tetra ReptoGuard Water Conditioner Blocks, Protect Your Pet From A Variety of Disease-Causing Organisms Each turtle-shaped block treats up to 20-Gallon Value Pack of 12 Blocks!