Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Slate should not alter the pH. If you already have the slate or if you can get a sample (broken bit is enough) you can test this.
Test the water you intend to use in the tank (or is already in the tank) for GH, KH, pH, TDS.
Then put enough of that water in a jar to cover the bit of slate.
Test again the next day, and every few days for a week or more.
Almost no change, or no change in a week or more suggests it will have no effect on the water in the tank.
If the parameters change in the test jar then they will change in the tank.
Use a plain black slate. Sometimes the word slate applies to almost any sort of rock that can be cut into thin squares.
Sandstone (gritty texture) unless it is really solid.
Slate with fancy colors that could be heavy metals. I have seen slate with a high level of iron in it. I would not be surprised at other metals. A little trace of iron is no big deal. By 'little trace' I mean a few dime-sized spots in a 125 gallon tank.
In the mean time keep the manzanita under water by weighting it down, perhaps in a child's wading pool or a garbage can.