Measured my tank this morning, looked closer to 6 than 8, so let's call it 6 mg/l. I think at this point I'm not going to worry about it, will recheck if fish count goes up.
What level would have caused you to worry?
I'd personally aerate some tank water for an hour or so nd measure it to see what full saturation reads in that setting.
Here's what I found for my tank system in the context of seeing what effect ozone might have. There was a drop at night vs day. I was using a big non-recirculating skimmer.
I obtained the following test results before and after using ozone in my system:
Before ozone:
Main 120-gallon tank just before lights on: 5-6 ppm O2
Main 120-gallon tank just before lights off: 6-7 ppm O2
Refugium 1 (always lit) just before main lights on: 6-7 ppm O2
Refugium 1 (always lit) just before main lights off: 6-7 ppm O2
Refugium 2 (always lit) just before main lights on: 6-7 ppm O2
Refugium 2 (always lit) just before main lights off: 6-7 ppm O2
After ozone:
Main 120-gallon tank just before lights on: 6 ppm O2
Main 120-gallon tank just before lights off: 7 ppm O2
Ozone tubing reactor effluent after activated carbon: 6-7 ppm O2
Sump just before lights off: 7 ppm O2
A strict reading of the values that I obtained suggests a small rise of between 0 and 1 ppm in O2 in my main tank. However, the kit is hard to read in the 5-8 ppm range, so the oxygen could have risen by 0-1 ppm, or more, or perhaps not at all. The effect, if real, may be much larger in an aquarium where the oxygen drops much more at night (i.e., those without refugia lit at night or those with less aeration).
