N-deficient? No way...it is the other way round instead. Something is producing ammonia in large amount that even the plants are not able to take up fully or the plants are pretty much stunted from lack of certain nutrients and are not taking up any. Do the blackout properly and make sure you get your plants going is the best solution.

Ammonia use for feeding plants? Nah, don't..it kills critters and the thing is algae loves them. Go with nitrate instead..