cr500_af;530182 wrote: But yes, just take a little lime powder and add enough RO to make a thick paste. If you are careful and turn off the flow in the tank, you may be able to nuke the hydroids and then after it has been in contact with them for ten or fifteen minutes, take a turkey baster and suction the paste off the rock. It will take you being very careful, and even then it could be a little risky. I've done that before, and I did slightly "get" a monti cap that was right there, but it recovered.
If you elect to try this, I'd avoid turning the PH that faces this particular rock on, until you give it some time and get all of the paste back off the rock that you can. Then you can restore full flow. You also have to be sparing with the kalk, as too much could spike your pH up.