[wet]

If you're reconstituting RO and old growth on your darkest green plants look like that, IME you're not adding enough Mg, for what its worth.

 

[wet]

Those are tank levels? Its safe to assume that however you're adding Ca, Mg, and Fe includes a "comprehensive" trace, yeah? If so, I think you should reset your troubleshooting and start over on the more obvious signs: CO2 (C is your biggest nutrient by far) and are the big leaves of a too tight bunching of H. Polysperma causing too much shade on old growth, and if its the latter but the tops look good and are sustainable, should you replant or just hide it with hardscape or a mid height grower .