If one of these conditions exist, a pond owner should inspect his pond daily for signs of oxygen depletion. If disturbed they dive but quickly return to the surface. If oxygen depletion has not reached a lethal level, fish are at the surface in the early morning but return to deeper water as oxygen builds up during the day. This may continue for several days. The pond owner should take corrective action immediately.
Fish being fed suddenly stop eating.
Testing the oxygen level in a pond is an excellent way to detect a developing problem. Oxygen test kits and meters are available for this purpose. Tests of oxygen levels in ponds should be made at dawn or shortly thereafter. This may be impractical for a farmer with a large number of ponds. One can estimate the possibility of an oxygen depletion by measuring oxygen in the early evening and again two or three hours later.