Growth rate of your fish will depend on a lot of variables including the type of fish. crowding, quality of food, water quality and water temperature to name a few. In our experience we find a lot of the published commercial growth rates to be overly optimistic for a more casually run home system. · It can take 9 -12 months before tilapia are large enough to harvest (about 500 – 550 grams). · Make sure fish are being fed enough food on a regular schedule. · Check for overcrowding. Keep only up to 1 kilo of fish for every 40 liters of water in the system. Or think of it as 1″ of fish for every gallon of water. · Check water temperature. Tilapia love warm water and fish grow slower at low temperatures.