Dissolved Oxygen Module- PM3
Many marine aquarists only give the oxygen content in their aquarium
water a passing thought, not fully understanding how the level of the
Dissolved Oxygen (DO) in their tanks can affect their entire tank
population.
In one way or another, everything (fish, invertebrates, corals,
algaes, bacteria) in the oceans and in marine aquariums react with
oxygen. Most of the living beings of this planet require oxygen in order
to metabolize nutrients. Fish, invertebrates, corals, and bacteria
ingest O2 and expel CO2 day and night.
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