Hard to see under lights, mag def has yellowin leaves and brown spots, so do a cple other things unfortunately. Have you fed it for mag? I'm gonna say from the off I'm not sure what it is but could probably help you narrow it down, or eliminate some things. 17c is about as low as it should go, I don't like droppin below 19c. Erm, what's ya water like, hard or soft? If it's hard it's full of calcium so if you've been constantly been giving calmag then it could be salt build up. Could be hungry for P K if you've not added any boosters.