Do you use 4 dKH water in the drop checkers - water made with distilled water plus bicarbonate of soda to raise the KH? "Plenty of flow" would be enough to see almost all of the plants swaying a bit in the current. So, you may not have enough circulation. And, yellow drop checkers should indicate enough CO2 to be killing off the fish. Is that happening?
In a high light tank, like you have, very slow growing leaves are very likely to get BBA growing on them, in my experience. Algae tend to grow on unhealthy or non-growing leaves. And, both Java Fern and Anubias have always been an algae problem for me when I had even medium light. Reducing the light to the low medium range will still allow almost all plants to grow well, with good CO2, but the algae problems will be much, much less.