mixing substrates

I'm getting ready to set up my first two cichlid tanks - one will probably have black moon sand (55gal - Malwai) and the other pool filter or cilica blasting sand (46 bow - Tanganika) - I've not found pool filter sand yet, but I've been looking.

However, I was considering going with crushed coral to help buffer the water. Our city water has fairly low ph out of the tap - 7.0 or 7.5 -

I have a canister filter for one of the tanks, is it the actual substrate crushed coral people throw into the canister? - If so, how much works to acutally buffer the water?

Also, I was thinking of mixing crushed coral with the sand - will that still buffer the water enough to make it worth the while if its mixed with sand? Has anyone done this before? - Not sure how the black moon sand would look with white chunks of crushed coral throughout it (I have another thank with white and black gravel mixed - I think it looks pretty cool)- would it bother the fishes ability to sift the sand in their mouth if there's chunks of coral throughout?

Thanks in advance for any thoughts on this...

Kerby