again you haven't mentioned the age--I have seen this curl in a couple f1's i grow WW being one of them--I use top drip for my mums and if yours are real young 3x 30 minutes should do it for water. up that RH to 50-55% if possible. If needed you may supplement with a LIGHT dose of a calmag product via foliar feedings or hand feedings a couple x/day.
I would recommend the use of h2o2-35%grade packaged as super oxy h2o2 its a fine product to keep the buckets crystal clear (1.7ml h2o2 per L nute soln every 3-4 days). It should be readily available at your local hydro shop or from med supply/lab equipment suppliers--packaged as 35%h2o2 and usually cheaper--also reduces trips to the hydro shops (don't know about you but I avoid those places like the plague)
H2O2 and Dutchmaster Gold 2 part is all I use in my veg room for an 8x4 tray with 70 mums-I have the occasional problem still with this same curl--BigBud and White Widow--The other thing is WW can be a bit nute sensitive so if it the only strain you are growing you can backoff the nutes with no negative effect see if that doesn't help--If you don't see deficiencies then you are fine. Don't worry about the lighter doses it is better than nute burn in WW--which it will certainly resent.
BTW if your tap water is under 200ppm you don't need to waste time or money frigging with store bought or filtered water. Mine is 140ppm and has never been a prob.
I have also assumed when you say Ph is good that you are calibrating your meter a couple times/wk.....if not please do.
Good Luck and again this curl is probably not to serious or crop threatening.
-OitW
