Plant problems.

I currently have got 4 plants that are in a nft hydro system. There is 1 plant that is suffering at the moment. I did a reservoir change yesterday, because I was having some problems with ph stability on this 1 plant. It had dramamtic changes from 6.3 to 3.9 overnight. I havent been able to get the ph dialled in on this grow and it does fluctuate, but not to that degree. Anyway, I am using the canna nutes range and its on veg. A friend suggested that I should give the H/W version of this brand a try and he game me some. Now the real questions are is the H/W version going to affect the plants in any way? The other plants are not showing any problems with the change in nutrient type. Also the lady in question is showing very bad signs of drooping since the change. Its been about 16 hours simnce change and it took bout 3-4 hrs after res change for me to notice the drooping. Also from what I have read is thst it is possibly showing signs of over watering, but how can you overwater a hydro system?. Im tempted to stick in an air line to increase the oxygen in the res to help this sick lady back on her feet. The ph is at 6.2 and the nutrients are reading at 0.7-0.9 ec.
Any help and ideas would be greatly appreciated