The natural process of nitrification breaks down ammonia-nitrogen (produced by the fish as a metabolic waste product) into nitrite and then nitrate. This process also produces acids that lower the pH of the water. The optimum pH to maintain is 7.0. This level is a compromise between the optimum level for nitrification (7.5) and the optimum level for lettuce growth (6.5). To elevate the pH to this level, base should be added on a daily or every-other-day basis. The best bases to use in the system are potassium hydroxide (KOH), and calcium hydroxide (Ca(OH)2). These are best because they do not increase the sodium salts in the system (which are toxic to the plants and can cause tip burn) and these bases provide essential nutrients to the lettuce plants. Potassium and calcium are required nutrient additions to ensure healthy plant growth.
Iron is deficient in the system and is supplied by additions of Iron Chelate (Fe 330) once every three weeks. The quantity supplied is calculated to have a concentration of 2ppm Fe in the system water.
We do not add any other nutrients. Nitrogen is an essential nutrient that is provided by fish waste products. Other leafy green plants grow well in this system because of the available nitrogen. The system has been sized so that the balance of fish and lettuce maintains a high nutrient level to the plants and adequate cleaning of the water by the plants.