Abstract: A saltwater aquarium environmental control system contains a salt mixing tank and has a specific gravity float switch to maintain proper salinity in the mixing tank so that a ready supply of fresh saltwater is available for replenishing water in the aquarium tank. The system may contain a second specific gravity float switch to monitor the saline concentration in the aquarium tank. The system provides for mixing replenishment saltwater, for automatically dispensing replenishment saltwater at timed intervals in predetermined volumes, and for monitoring and adjusting proper salinity in a saltwater aquarium. A routine exchange of saltwater into and out of the aquarium tank is maintained in order to maintain clean, fresh saltwater in the aquarium tank at all times.