Yeah, that looks like limestone. no clue on what's going on in the 1st and 3rd rock, but the 2nd looks like a brachiopod. -thanks for posting pics.
As what people said above, odds are is that your aquifer is limestone, so the water out of the tap is going to be nearly saturated with carbonates.
What the rocks will do is buffer the water, and keep it at that higher pH.
Unless you have highly acidic tanks, I don't think there will be any noticeable degrading of the rocks.
And even if they did degrade, I don't think they would crumble as much as they would just look more weathered, cracks/fissures would widen, edges get smoothed out, etc. (just a guess)