My tap water has kH of 7-8 and gH of 11-12, pH 7.8. I'm starting a new tank and looking around for some rocks. I was at a landscape place the other day and found some nice stuff but they didn't know what the source was. So, I don't know it they are limestone or not.
If they are limestone or some other carbonate source, how much of an effect will they have on my hard water?
I'm testing some samples but I really want to scape this tank and get moving!!!
If your water is already that hard calcium carbonate rocks won't make the water harder at all, Calcium carbonate only acts as a buffer when the hardness and ph of the water drops real low, That's why us reef guys don't have to use sand in our saltwater tanks because it really doesn't do anything for buffering unless the ph drops to low(which should never happen), Now if your after real soft water then I would stay away from any of those rocks but they won't affect anything if your water is already hard.
Not a problem. As noted by DSP the calcium carbonate types of rock will only dissolve in water that is acidic to neutral. It does not dissolve very fast in harder water. Just enough to keep the parameters stable.
This is why it is well thought of for marine tanks and hard water tanks such as for Rift Lake Cichlids and certain other hard water fish.
If the water ever does change, the rocks change it right back to where it should be for these types of tanks.