I got the same issue and I'm in DWC and was ph'ing at 6.0 when I got the def. I dropped it to 5.5 and it's not spreading anymore but looks like only a very tiny bit of recovery each day...try dropping that ph a bit and look up zinc Deficiancy I think it gets locked out at 5.7. Hope you solve the prob I'm still waiting for a little more recovery till I flip em!