Work on the pH. The 5.2pH is most definitely not "perfect."
Somewhere between 5.5 and 6.0 is the range to be in. As close to 5.8pH as possible all the time would be better. Mix your nutrients 24 hours in advance to give them time to rest and balance. Don't run an air stone in the pre-reservoir before adding nutrients to the main rez.
I think the pH is probably the thing to balance out here. Adding or subtracting elements won't fix anything if the 5.2pH is the problem.