Are you watering deep enough to provide moisture to the lower root zone?
Does you medium absorb and hold the moisture, or does it flow right through?

Are the leaves dark or light green? Are the leaves paper thin, or hearty?

What are day/night ambient temps, (room temperature, not under the lights) and what's the room's humidity?

Phosphoric acid is water soluble, and will easily flow-out with the runoff. Doubtful it's the cause. Were you giving too much phosphorus, the ph would crash before any indicators of build-up would occur.

After this I think filling-out the troubleshooting form would be appropriate, along with any pictures you can provide.