Is this an over-watering issue? nute problem?

I have had this issue once before, but it was with one plant and in soil. I am now in hydro, and this problem started with 1 plant at 13 days into flower, by day 16, 7 out of 9 are having the same issue. the leafs are deep green in color, shine more than usual. the curl under is fairly stiff. some buds have started showing brown tips on some hairs, but they aren't close enough to any light to burn. I grow in Coco/ on coco mat, lights are all 20inches away, temp is 76 degrees (+/- 1). Water once a day.
any thoughts?

nitrogen problem maybe

I thought that at first, but without yellowing or any other leaf indication, I was doubting it.

Not a nitrogen deficiency but toxicity.

yeh , too much , I`d just do 1/4 strength , after a few plain waterings

Thanks for the replies. I will give that a shot

cleaned and bleached the reservoirs last night, fed the plants straight water, and already this morning they were showing improvement. Thanks Again.

Go Blue.