[TimT]

My suspicion would be a bacterial overgrowth. Which would in turn cause low oxygen levels.

Most bacteria are fed by carbon sources such as sugars, acetic acid and alcohol.
Do you use any acetic acid(vinegar) or alcohol near the tank or in the house?
Do you use any additives to help coral growth etc as they can sometimes have sugars in them.

What are your tank parameters for calcium and alk?

Is there a lot of detritus in the tank?

Cheers,
Tim