Update, 1/30/21
Bad news, and good news.
Bad news: it looks like I have dinoflagellates in my frag tank.
View attachment 23673
Good news: I have dinoflagellates and this lets me perform some
a priori hypothesis testing.
Hypothesis: I have been feeding two to three cubes of spirulina-enriched brine shrimp and mysis daily for a medium kole tang, a clown, a sixline wrasse, and a couple peppermint shrimp. Upon further reflection, my skimmer has not been pulling a lot of skimmate out. Based on a combination of low bioload and low skimmate, I am reasonably certain I have low phosphates and/or nitrates, since dinoflagellates thrive under limiting nutrient conditions.
Results: Nitrates were at ~20 ppm. Phosphates, however, were at 0.01 (and the Hanna ULR Phosphate Checker has an error of +/- 0.02, so entirely possible it's zeroed out).
Conclusion: Hypothesis confirmed!
So, now that I have a likely contributing factor to dinoflagellates, I can make a plan of attack:
1. The dinoflagellates I have are ostreopsis. None of my CUC has died, which is good, but they're thought to be one of the more toxic species. So: keep protein skimmer running to remove dead dinos, and run extra carbon to remove toxins.
2. Feed more heavily to up phosphates. This includes feeding Reef Roids every day since, in my experience, that increases phosphates pretty quickly.
3. 3-day blackout. Dinos are photosynthetic, bacteria/other microorganisms aren't. Make my tank less hospitable for dinos while encouraging other bugs to grow.
4. Dose Microbacter 7 every day to increase the amount of competition against the dinoflagellates.
5. If I can find one, hook up a UV sterilizer, since ostreopsis live in the water column and a UV sterilizer would help kill them.
I'll post an update in a few days once I have answers!