Leaves 'rusting', nute burn?

900+ ppms way too high for their age, IMO. I have 'seen' 3-4 footers in DWC with max ppm at 1000 during height of flowering. you r getting lockout from high fert levels. dont forget to flush the rockwool cubes as well. the problem I have seen many many times is once the roots have grown enough to reach the res water, you need to stop feeding from the top b/c they will drown. the cubes need to be dry once the roots hit the water, not wet.
make sure your res temps do not exceed 70. Botanicare make a 10 combo micro benes for rhydro res called Guardian TH. works wonders keeping root rot in check.

h202 is everyday hydrogen peroxide it will help to oxidize any dead roots or other organic material and it kills any undesirable anaerobic bacteria in your resevoir.