i use RO water. but i think the problem is the contents of the tank, not the water source. the water's pH was neutral.With so high pH you probably have high KH too. You need to bring the KH down first. Easiest way is to mix RO water with tap water during water changes to get reasonable parameters.
You can choose fish that will thrive in your hard water too and never worry about adjusting parameters.
Pumice usually forms from high-silica lava that is low in calcium and magnesium. Chemically, it's perfectly safe for a tank, unless lime has been deposited by groundwater in its pores. You can test by putting a little strong acid on it and seeing if it bubbles. You can also get rid of any lime residues by soaking it long enough in a strong acid.But that may be due to my lack of experience with pumice! It's volcanic and I tend to stay away from volcanic types as there is no real way to say what was melted and left in them.
I've never heard of anyone having issues with pumice or other volcanic rock in an aquarium. One of the more common use for pumice is in the aquarium for biomedia. I have bags of it in my canister filter.I was not familiar with pumice stone for aquariums, so did some reading and now I might have some real questions of their value for this purpose. Is this something that you have used before without trouble or a new item? I find one of the uses for pumice is making concrete blocks and that says very alkaline to me.
I might think that the pumice is the problem and other subs would work better. Trying to get the PH down while using a highly alkaline sub would not be one I would want.
But that may be due to my lack of experience with pumice! It's volcanic and I tend to stay away from volcanic types as there is no real way to say what was melted and left in them.