Are you referring to crushed oyster shell?
I know many people refer to it as grit but it isn't grit.
Grit should be insoluble stones that aid in grinding food in the gizzard.
Oyster shell is soluble. It breaks down quickly after being mixed with digestive acids in the proventriculus.
I don't advise to mix oyster shell - or anything else with feed.
They need to opt to eat the OS if they feel the need for the calcium.
Are you feeding a layer feed? Are you feeding many treats?
Drastically changing the ratio of calcium, phosphorus, magnesium and vitamin D3 can have deleterious effects.
Just the imbalance itself, rather than insufficient calcium can cause shell quality issues.
Edited by ChickenCanoe - 11/16/15 at 3:18pm