Young Macdonald's Farm Dissolved Oxygen Dissolved oxygen (D.O.) limits the amount of feed the cultured species can metabolized, use density of the activities of the biofilter. We try to maintain DO at 5 ppm. It will fail below this level occasionally but usually this causes no harm if it persists for short periods of time and other parameters are in good order. DO depressions occur mostly in the hour after feeding and the more feed used the greater the variation. Good feeding practices of spreading the daily ration over the greatest number of feedings possible will reduce these depressions. (Example: 3#/day could be divided into 6 feedings of 8 ozs. each.) Automatic feeders will take this concept to its ultimate level of spreading food over the largest period the longest period possible. So if you are feeding by hand, DO should be checked before feeding and about 30 to 45 minutes after feeding. The biofilter oxygen requirements are not as high and about 2ppm is probably satisfactory RBC, in the demonstration model, it is not a problem as the bacteria have access to atmospheric oxygen. One must also keep in mind that DO is consumed by many processes in your system other than the cultured species or biofilter, some of those are chemical, some are heterotrophic bacteria and algae under low light conditions. So all oxygen is not available to your fish. NOTE: SEE - Remote water quality management