FishingRod,
I am not sure if your reference to impending crash line refers to oxygen depletion, ammonia concentration, other factors, or all or many. Anyways did want to contribute some context because I know you will appreciate it.
Fish use oxygen but this isn't where the risk of oxygen depletion comes from. Usually this risk either deals with excessive plant respiration at night or deals with the oxygen consumption related to decaying organic matter (like an algae or plant die off or the mixing of accumulated organics previously stored in an anoxic bottom layer). The standing weight of autotrophs usually always greatly exceed the fish weight and so they influence DO drawdown more than fish do. Also bacteria probably almost always have a greater standing weight than fish do depending on the load of decaying organic matter they have available for consumption. Generally speaking, the conditions that would cause fish kills would happen without the fish oxygen demand. IOWs managing oxygen depletion fish kill risk by managing the numbers or biomass of fish is not an effective measure.
