Aquaculture water treatment systems use bacteria to convert ammonium and nitrite into nitrate under aerobic condition, while nitrate removal can be accomplished using sophisticated denitrification systems under anaerobic condition (van Rijn, 1996; Abey- singhe et al., 1996). Menasveta et al. (2001) described a closed, recirculating seawater system with denitrification for shrimp broodstock culture. However, denitrification systems are complicated and impractical for large-scale shrimp culture. Presently, these systems require oxygen reduction to almost zero before denitrification begins and automated controls during denitrification.