OK, I'm pretty sure I've overlooked something simple to explain why my submerged plants aren't growing: gas exchange. This tank is covered with floaters, and I've intentionally kept surface agitation low to keep both the floaters and the apistos happy. My sponge filter is running at about 10 bubbles/second, and the outlet is protected with a feeding ring to contain the surface agitation to 10x10cm. So that must mean I'm starving my submerged plants of CO2. I will remove the feeding ring and half the floaters. Hopefully that will move the needle. I also have liquid CO2 (Excel) around to spot treat BBA. Maybe I can dose that as well.