Nitrogen deficiency question

[PerceptivePesce]

How long does it take for a nitrogen deficiency to manifest?

I'm dosing my tank with expel-P which also requires lights-out for 24 hours.  I've done 2 treatments 8 days apart and my 3rd treatment is scheduled in 2 days.  I've also lost 2/3 of my nitrogen producing guppies, and I've done 2 extra WCs (before treatment WCs were performed once a week, I've done 2 WCs per week since treatment.)

My java ferns were fine before the treatment, but now they are dying.  The leaves are quickly rotting away.

[Scapexghost]

I think this has more to do with the light than the nitrogen

[Mmiller2001]

Depends on the plant. Slower growers will take longer than fast growing stem plants.