Light and CO2 look good.
Could be N, as you are describing a possible mobile nutrient problem. However, can you paint a full picture, first:
- Current NO3, PO4, GH, KH, pH and TDS readings?
- What you are dosing (product and quantity) and how often?
- Substrate type and how long has it been in place?
- What is your filter setup?
- Cleaning regimen (filter and water change frequency and amount)?
- Circulation: surface rippling and are all plants gently moving from top to bottom?
- What is your water source and do you use a water softener?