Both freshwater and seawater should be treated before usage. Initially, the water should be allowed to settle and filtered to remove suspended solids and parasites or predators, including eggs and larvae. New & Singholka (1985) described filters suitable for this purpose, including a simple beach filter. Valenti (1996) recommended passing water through a gravel/sand filter to remove larger particles and then through a series of cellulose filters with a porosity of 5 and 2 |im. Subsequently, the water requires disinfection using UV light (Daniels et al. 1992), ozone (Valenti 1996; Reddy et al. 1997) or chlorine and/or formalin (New & Singholka 1985; Carvalho & Mathias
1998). Some hatcheries use a combination of the above methods.
Several problems caused by residual chlorine and ozoni-sation have been observed. Thus, the use of ultraviolet (UV) sterilisation has been the best choice. However, chlorination is more frequently used. In such cases water is typically chlorinated for 1 day with 20 to 50 mg/L of sodium hypochlorite (2-5 mg/L of active chlorine) or calcium hypochlorite with the same concentration of active chlorine. Sodium hypochlorite is more persistent and not so easily removed by aeration, therefore calcium hypochlorite is preferred. The residual chlorine is removed by aeration for at least 6 hours. If chlorine persists or in case of emergency, residual chlorine may be removed within a few minutes by adding sodium thiosulphate in the mixing tanks at a concentration of 3 mg/L for each 1 mg/L of chlorine. Caution should be exercised in the use of sodium thiosulphate because it has been shown to be toxic. Stage I freshwaterprawn larvae have exhibited mortality at levels of 100 mg/L sodium thiosul-phate (W. Daniels, unpublished data) and may be stressed at lower levels. It is essential that the water has no residual chlorine before its use in larval rearing. In Thailand, 40 to 50 mg/L of formalin is sometimes used to eliminate protozoa and bacteria from the seawater used to prepare brackishwater. The choice between chlorine and formalin may depend on local cost-effectiveness and legality. Further details of water treatment can be found in New (2002).
After disinfection, both freshwater and seawater should be stored separately in auxiliary tanks shielded from light to avoid algal growth.
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