Your critters are not the only things that use oxygen in your tank. Organic materials (i.e. Dissolved Organic Compounds, uneaten food and fish detritus on the bottom of the tank) consume a lot of oxygen as they break down. And now it’s even popular to add nitrifying bacteria additives which multiply very rapidly, consuming a large portion of the available oxygen in the tank water.
Of all the organisms in your aquarium, large fish are the most acutely at risk to low O2 levels. In some situations with heavily loaded systems, simply the failing of a skimmer can bring the O2 levels down to dangerous levels and cause a chain-reaction fish die-off . This is where the PM3 Dissolved Oxygen module comes to the rescue.