Hey there, Sean1. Circling back to the matter of oxygen, have you had a chance to tinker and determine whether your levels are what you want?
I finally got my little test kit in and my tank did appear to be on the low end. Not bad, though. Sister Golden Tail Surprise (our damsel fish) never gulped at the surface or anything, but our levels were at a 6 when the ideal would have been a 7 or 8, and octos of course are better off at the higher end.
I've since been experimenting with ways to boost the O2 and testing along at each attempt. Nothing I tried has had any impact: bubbler in sump, chaetomorpha in fuge, other plants in tank (and fuge), and using phytonutrients to boost 'pod populations. Parameters have remained stable for pH, ammonia, and nitrites/trates with the exception of a bump in nitrates when adding the phyto slurries, though that has since stabilized. Other than that, I haven't added anything to the tank--no buffers or dosing of any kind. No one in there except Sister and a host of small inverts. Have also tested at various times of day, as well as testing while light was on for 1+ hour and then off for 1+ hour.
Anyway, the iterative but fruitless experiments continued for a few weeks until I finally just walked off in a huff. Now, after doing nothing but a weekly water change over the holidays, the oxygen is reading at 7-- up a full point. How very cute. (Not!)
A few other notes:
- The test kit
I don't actually know whether my test kit is even worth a dern, nor have I any faith in my ability to compare clear pink water to printed pink blocks on card stock. The pink water never seems to match any of those blocks! However, even though I may not be picking the correct color on the scale, it's probably safe to say that the color has intensified, which indicates that the oxygen levels, whatever they truly are, are denser.
- New tank
My tank is in "late cycle stage", which means that it actually finished cycling months (and months) ago and has since been fit for fish, inverts, etc. But it's still a new tank. Maybe it's just one of those things where the longer it runs, the greater the ability to support oxygen densities, other gas exchanges, and better living through biochemistry.
- Clumsy water changes
I've sometimes let my saltwater mix sit for 1-5 days before using it to change tank water. I don't know what the true limit is on letting a salt / RODI mixture sit around before it's growing bacteria and is no longer suitable for the tank-- or whether it has impact on O2.
- Skimmer died
We had a protein skimmer that died of old age a couple of weeks ago, right around the time I was adding plants. While the O2 levels didn't seem to be impacted one way or another, it's possible that we'd have seen a bump in O2 sooner if the skimmer had been spinning the entire time.
- Circulation
Circulation in my tank is concentrated at the surface. There are probably stagnant zones toward the base of the live rock. Once I improve circulation in there it may also impact oxygen density.
So on the whole, I still have absolutely no idea how to regulate oxygen levels in my tank, but it may just be one of those things that works itself out on its own. If you've discovered anything on your end I'd love to hear about it.