What would crushed coral do...

if I placed it into the chamber of sump where overflow first enters? Would this help buffer and add calcium besides a bio home?

 

Ah nope, the pH of the water is not low enogh. Dissolution is around pH 7.7 for aragonite. If there is a bunch of fine dust or small points on the surface grains, it may buffer for a very, very short time. With that being said organic films and bacteria will still grow over it and any furhter buffering will be gone.

 

Makes sense. What about cruches oysershell. Same thing? Guess if there was a calciumk based substrate that would slowly leach calcium then we wouldn't need dosers and such, right?

 

Correct