Is there any other test I should be doing to see if a rock is "fit" for the tank?
I added two to a tank I'm cycling. Ill check the PH against another tank to see if there is any swings.
Aside from the acid test, not really. limestone is known to effect water hardness pretty significantly, so maybe watch out for limestone, otherwise most rock is likely to be pretty inert. When I went to the landscaping supply I brought a little 2oz spray bottle of vinegar and sprayed what I was looking at lol. Didnt even get funny looks.