-TK
This needs to get fixed before you start generating root mats on the bottom of the root chamber. Pull off the brown bits - they will not recover and just make more food for fungi. Bump the Zone to 2.5 - 3 ml.
-AK
I keep the Zone elevated from base level until the problem stops. I then go back to base at the next res change.
If I have a problem with a unit and I'm adding more ppm of Zone than usual, by default the ppm of nutrients goes down and this inhibits growth. So as soon as the problem is averted, I try to change out the res and get everything back on track.
For me its not a formula of how many days to add back Zone. I stick to base and biweekly res changes unless there is a problem. If there is a problem, I elevate Zone until its under control. I will change out a res completely if there is any question.
So - bump it up to contain the problem. Once its under control and the source of the problem identified - res change to base levels. If the problem returns, raise the base level of Zone until the source of the problem is resolved. Any time the res is going to be warmer than usual I bump my base from 50 ml to 60 as a preventative.
One other thing to check would be spray lines and drain tubes. These machines are all about dissolved O2. High DO prevents rot. If the throughput of the nutrient solution is not high enough - more rot happens.
I hope this helps. Please stay in touch on this. It can be very frustrating to get everything back in balance. Mystery critter problems are the worst. But once the battle is won, the plants will take right off again.
-AK