Closed loop systems are self-sufficient systems that produce their own food, and still have room to harvest fish and crops for you. The goal of a closed loop system is, primarily, to not have to BUY anything to keep them functioning.

A Closed loop system may involve harvesting and feeding by the owner, from crops or livestock grown in separate tanks maintained for that purpose, or it may be something that allows a balance of feed crops and harvestable crops and livestock. The degree to which the system self-maintains can vary depending on the elements in place. Ideally though, there is no cleaning required, and no feeding required, though water levels always require monitoring, and water must be added as needed due to evaporation and consumption by the plants. A good closed loop system will function like a natural lake or pond, with fresh water coming in from the output end of the plant tray, and dirty water exiting through the pump to enter the intake end of the plant tray. The tank is cleaned through a combination of algae eating fish and crustaceans, scavengers, and the plant filtration.

Closed loop systems typically require layering - algae and plants in the water, herbivore fish or crustaceans which serve as cleaner fish and feeder fish, omnivore fish which may partially serve as feeder fish, but also provide a small pan fish, and carnivore fish which are top level predators and will eat anything small enough to fit in their mouth, but which provide you with a larger harvestable fish. The plant systems on the top may be similarly layered, with the plant trays organized so the plants do their job more efficiently, based on the type of plant and tolerances.

IMPORTANT NOTE: Water in closed loop systems MUST be free of Chlorine, Chloramine, and other additives. If it is put in to kill bacteria, it will KILL FISH. If it does not kill them, it can inhibit growth, and it ALWAYS inhibits growth of algae, which is necessary for a closed loop system.

Use a filter, and then heat the water to tank temp (it is a BAD idea to run hot water, or even warm water, through a faucet filter, it causes various problems with the filter that shorten the life of either the cartridge, or the filter itself).

Alternately, you can boil faucet water for 10-15 minutes, and then cool it to tank temp.

A closed loop system maintains a pretty delicate balance, and chemicals in the water that interfere with growth of either FOOD SOURCES (Algae, plant, diatom, fish eggs, fish fry, shrimp larvae, etc), or the harvestable fish and crops, will either cause the system to collapse, or they will keep you from ever being able to harvest anything, or even from being able to stabilize the system.