Any water system with plants and or animals has the potential for BOD to be more than supply. The more living organisms and the more static the water the higher the chance. That is the exact reason overwintering tilapia can be a problem (to much biomass - BOD - for the available supply). It is simple - a water system is like a car engine - you can run it a long time at low RPM but if you run it at high RPM it is likely to break sooner.