Good news, bad news.
My fish had been hiding more and more over the past few weeks. They were to the point that I would seldom see the bristle nose except at water change time. Occasionally in the mornings, some of the fish would look ill - darting around, trouble staying up right, etc. Turns out that my CO2 was too high.
I've read about paying attention to the fish. I have notice slight changes, but didn't not associate it with low O2. Most of the fish looked good, most of the time... Cardinals, apparently, are my canary of the aquarium - they tolerated the CO2 the worst.
My CO2 is controlled by a PH meter, about 1 degree of PH below the gassed reading.
Today, I added an air stone and man did the fish come alive. The panda cory's are bouncing off the wall. The cardinals are swimming more. The bristlenose are out and eating...
I'm going to keep the air stone running and the CO2 at about 1/2 degree above where it was set before. The drop checker was a nice green - no blue, no yellow. New 4dKH and reagent solutions. I'll see where it sits overnight with the air stone.
Now, time to see about the algae... Even with dosing KNO3, my nitrate levels were still very low ~2 ppm while the phosphate was also at 2 ppm. I will be dosing additional KNO3 and watching the readings.