If dosing Neonitro is the only change you made, my guess is that the added nitrate increased the growth of certain bacteria(or possibly some other organism), and the bacteria(or other organism) consumed more ammonia and caused the algae scrubber not to have enough ammonia. If I were you I would dose ammonium chloride at least while the algae scrubber light is on. I think the ammonium will stimulate the algae to grow a lot(if you add enough and at the right times), and also consume nitrate and phosphate, obviously. If you had very low nitrate before you added the Neonitro, I believe that adding it could have definitely stimulated the growth of some organisms, and caused a reduction in ammonia.