There is a trend in aquaculture towards reducing water exchange in ponds. The reasons for this are

• high water exchange rates increase the risks of introducing disease, toxin or toxic algae to the ponds

• the water being pumped into the ponds may be polluted and indeed may often be worse quality than the pond water

• environmental regulations continue to tighten and restrictions on discharge of farm water into the environment may be enforced

• low water exchange plus good pond management produces more stable ponds

• low water exchange heterotrophic ponds using organic carbon inputs are becoming more popular as managers learn more about pond ecology: see here for more information

• low water exchange conserves valuable slow growing organisms such as nitrifying bacteria.

Water exchange has been the traditional means of controlling water quality. Much of the effect of water change is due to the dilution and removal of PO₄ from the ponds. Phoslock is an alternative to water exchange and so is proving extremely useful for those farming operations that are keen to reduce water exchange.