Turface and SMS will remove the carbonates from the water. I have stopped that with coral sand, mixing something like 50% Turface with 50% coral sand. I use that for a Lake Tanganyika tank. You could also mix in oyster shell grit (sold for caged birds like Budgies) or limestone sand or very fine gravel.
I'm getting ready to change the substrate in my 75 gal to the Safe-T-Sorb which I think will remove carbonates also. ??? I thought I read soaking it first would help that problem ... does it? I soaked it for several weeks.
I had already bought some oyster shell grit in anticipation but now thinking if the soaking doesn't help much that it's no where near enough since you're using 50/50 mix Turface/coral sand. I wasn't planning on mixing it but rather putting it in my filters. Is that even going to work as it sounds like it requires way more than I'll have room for. I've got a test kit coming but right now have no idea what my kh or gh is. So I guess I'll just have to wait for the kit, do some testing and go from there.