Pool Blocks ALG-PB-2PK 600g Algicide (2 Pack) eliminate all forms of green, mustard and Blackspot algae in all fiberglass, vinyl, and concrete swimming pools. Pool Blocks are a slow released monthly tablet that work to kill and prevent Algae from ever forming. Once all algae is eliminated from the pool, less chlorine is needed to maintain sanitizer levels as the chlorine is free to perform its principal task of killing bacteria. Salt chlorinator and filter running times are reduced saving electricity as the water stays sparkling clear as all algae is eliminated even if the chlorine level falls.