Drain design important for tilapia in tank

Drain design is another important aspect of tank culture. Center drains are required in circular tanks for effective removal of solid waste. Water level is controlled by an overflow standpipe placed directly in the center drain or in the drain line outside the tank. A larger pipe (sleeve) with notches at the bottom is placed over the center standpipe to draw waste off the tank bottom. The sleeve is higher than the standpipe but lower than the tank wall so that water will flow over the sleeve into the standpipe if notches become closed. When an external standpipe is used, the drain line must be screened to prevent fish from escaping. To prevent clogging, the screened area must be expanded by inserting a cylinder of screen into the drain so that it projects into the tank.

Aeration requirements depend on the rate of water exchange. If water is exchanged rapidly, one to four times per hour, in a tank with moderate fish densities, aeration devices may not be required. The oxygen supply will be renewed by the DO in the incoming water. A flow rate of 6 to 12 gallons/minute is needed to support the oxygen requirement of 100 pounds of tilapia. DO, which should be maintained at 5 mg/liter for good tilapia growth, is the primary limiting factor for intensive tank culture. Flow- through systems should ideally be located next to rivers or streams to take advantage of gravity-fed water supplies, but pumping is practical in many situations. Limited water supplies frequently restrict exchange rates to a few times a day or as little as 10 to 15 percent per day.

In this case, aeration is needed to sustain tilapia at commercial levels. Paddlewheel aerators, agitators and blowers (diffused aeration) are some of the devices used to aerate tanks. Aerators are rated according to their effectiveness (pounds of oxygen transferred into the water per hour) and efficiency (pounds of oxygen transferred/horsepower- hour). Aeration requirements can be estimated by using aerator ratings and oxygen (O2) consumption rates of tilapia, which consume 4.5 grams O2/100 pounds of fish/hour while resting and several times more oxygen while they are feeding and active. For example, a tank with 1,000 pounds of tilapia would consume 45 grams of O2/hour at resting, but maximum oxygen consumption may be at least three times higher (135 grams O2/hour) depending on water temperature, body weight and feeding rate. Aeration efficiency (AE) of diffused-air systems (medium bubble size) ranges from 1,000 to 1,600 grams O2/kilowatthour under standard conditions (68°F and 0 mg/liter DO). However, AE declines to 22 percent of the standard at 5 mg/liter DO and 86°F. Therefore, AE would range from 220 to 352 grams O2/kilowatt-hour under culture conditions. Dividing the maximum oxygen consumption rate (135 grams O2/hour) by the median AE (286 grams O2/hour) gives 0.47-kilowatt (0.63-horsepower) as the size of aerator needed to provide adequate DO levels.

A current trend for intensive tank systems has been the use of pure oxygen for aeration. Oxygen from oxygen generators, compressed oxygen tanks, or liquid oxygen tanks is dissolved completely into the culture water by special techniques to help sustain very high fish densities.