Having just done the RMS witch hazel treatment I can offer some observations that may be of help to those considering it.
Back ground:
Off and on STN of about 10% of my colonies in a 150 SPS dominant tank.
Alk runs 8.5-8.9
Ca 425
Mg 1400
NO3 15
PO4 0.10-0.15
The nutrients are checked with a hanna.
pH typically runs 7.95 on low end,
tops out at 8.30ish on high household CO2 dependent.
Did a 10 day TX used the bio digest in the am
Witchhazel at night when lights were off
Did the double first dose.
Observations: pH dropped between .10-.15 on both ends see photo, easy to see when treatment ended.
NO3 dropped to 5 a week in
PO4 dropped to 0
STN appears to have been halted as no new areas are affected and no new colonies affected.
Interesting is that frags of affected colonies seem unaffected by the STN(less likely a nutrient issue IMHO).
Take away watch the nutrients and pH and expect them to drop.