I decided to perform one more test. This time taking a bigger risk by doing it in my main tank with more diverse population, instead of smaller ones. It's a 46G, high light and heavily planted, with 30ppm CO2.
A bit of staghorn, mostly on old and deteriorating vesuvius sword leaves that should be removed anyway; a little more on lower light plants that got too much light and flow. I'll be interested to see the effect (and will definitely trim the plants later). But this is primarily to validate my earlier results in regards to the suspected respiratory effects of AlgaeFix; and the utility of using smaller, more frequent doses to give fish time to adapt.
I removed the bamboo shrimp and Marimo balls. Remaining livestock is guppies, swordtails, cories, neons, otos, cherry barbs, many red ramshorn snails, a golden loach, and a juvenile Ryukin.
4:00pm: Measured out a full dose of AlgaeFix as recommended on the bottle, 4ml. The plan is to add it 25% at a time, with three hours between each fractional dose. Added first fractional dose with a dropper, 1ml.
7:00pm: No adverse effects observed, added another 1ml.
7:15pm: Almost all guppies now staying at surface of water, some gulping air. Clear respiratory distress. CO2 is still 30ppm, oxygenation should be fine since I'm already seeing heavy pearling. Also, the swordtails are hiding. Did not see this in any of my previous tests, with fractional doses spaced further apart. Too much, too fast?
8:00pm: Quick recovery. Most guppies and swordtails now behaving normally. But one guppy is looking worse.
10:00pm: All now behaving normally except that one guppy, which is now having severe trouble maintaining normal orientation and buoyancy. It seems to want to stay at the surface, but it only goes periodically in bursts of effort; then soon sinks back down. This is a known weak strain, I previously lost two of its siblings due to a 70% water change. I hate to abort the test on account of this. I also hate to lose this guppy as it's extraordinarily beautiful. So as a compromise, instead of adding the next fractional dose, I turned off CO2, redirected wavemaker to water surface to more quickly reduce the level, and called it a night.
2:00pm: Turned CO2 back on, and returned wavemaker to normal orientation.
4:00pm: Lights on. The guppy has seemingly made a full recovery and adapted; it just took extra time, plus a reprieve from CO2 to help it past the crisis. Knocked on wood, and continued with the test. Added fractional dose #3 of 4. The staghorn looks very slightly diminished, could be my imagination. But also see some fish picking at it, which doesn't normally happen.
7:00pm: No adverse effects. Added final fractional dose.
1:00am: Lights out. No adverse effects seen at any time today.
Well.. It didn't go exactly as planned. Respiratory distress occurred, despite spacing the fractional doses by what I thought was an adequate amount of time.
It seemed to be species specific. Only the guppies and swordtails were affected. Basically, all the livebearers. None of the egglayers were visibly affected, even the scaleless ones normally considered sensitive.
Had I dosed the entire amount at once, I expect the instantaneous stress would have been enough that I'd need to abort entirely with a large water change to prevent many deaths, maybe losing some anyway. And then my lady probably would have killed me, too. Dosing fractionally saves lives!
Will take a few more days to see the effects on algae. I'll post a follow-up with that, and anything else that might occur.