Aim: to guide maintenance of optimal water quality throughout larval rearing and hence reduce stress and improve larval growth and survival.

• Limit the stocking density of the larvae to that suitable for the style of culture employed (50–100 nauplii/litre for single phase culture, or 100–150 nauplii/litre for 2-phase culture, transferring PL at PL4–5 to another tank for on-growing to PL15).
• Use only treated (filtered and disinfected) sea and freshwater for stocking tanks (see Section on Treatment of seawater for hatchery use).
• Freshwater should be filtered (as for seawater), come from shallow wells (although in Khulna such water is often bad, and filtered fresh surface water may be preferable) or drinking water supplies, since deep well sources may have high levels of hardness and/or heavy metals that are unsuitable for larval shrimp. Take water samples and analyze for heavy metals if concerned. If concerned about pathogen infection (i.e. presence of wild crustaceans in freshwater source), then disinfect with chlorine (as for seawater).
• Disinfect tanks, pipes and larval rearing rooms before stocking. Tanks, pipes, walls and floors may be disinfected by soaking in a 30 ppm active chlorine solution, then washing. Pipelines may alternatively be disinfected by pumping 10 percent HCl, 30 ppm active chlorine solution or supersaturated seawater through them and leaving overnight, before flushing out with clean fresh or seawater.
• Use batch (all in-all out) system by stocking each hatchery unit rapidly (preferably <4 d), then disinfect and dry (preferably with direct sunlight) for 7 d prior to next stocking.
• Use only highest quality and clean formulated feeds and disinfected live feeds for feeding. Do not overfeed, but check feed levels and larval feed consumption twice daily and feed only to the demand of each tank.
• Maintain continuous aeration at a level appropriate for each larval stage to provide sufficient oxygen and maintain feed and larvae in suspension. Aeration should be no more than half strength for delicate nauplii, with the aeration being turned up to full by the PL stage, where it is required to maintain the heavier feed particles in suspension and fully oxygenate the water.
• Routinely (daily from mysis stage onwards) cut off aeration for 10–15 min, check for excess build-up of sediment/uneaten food and faeces on the tank bottom and if found, siphon out wastes with a 2 cm flexible pipe (through a mesh to retain siphoned larvae), before turning back on the aeration.
• Change water on a routine basis, but only as much as required depending upon water quality. Typically only add water/algae during zoea, change 10–20 percent/d during mysis, 20–50 percent/d during early PL and 50–100 percent/d during latter PL stages to demand (See Table 5).
• Add EDTA at 5–20 ppm (increasing the dose with increasing presence of heavy metals) at each water exchange to help chelate heavy metals and kill bacteria.
• Conduct daily checks on water quality (using test kits or a specialized laboratory) to ensure that optimal levels of all crucial parameters are being maintained.
• Maintain temperature at 30 °C ±1 °C, salinity at 30 ppt ±1 ppt, pH 8.4±0.2, alkalinity 150 ppm ±20 ppm, and unionized ammonia and nitrate at <0.1 ppm each.
• Conduct routine (preferably daily) microbiological analysis (using TCBS and/or marine agar) to assess the evolution of the bacterial populations in each tank, to help decide on any action/treatments required.
• Screen larvae for viral diseases (if possible) throughout the larval rearing cycle to ensure their continued disease-free status.
• Make routine daily use of probiotics and (when required) disinfectants to promote water quality, limit pathogens and enhance the populations of beneficial bacteria.

Table 5. Guideline water exchange schedule for a 40 tonne larval rearing tank.