Oyster shell is not particularly soluble, I wouldn't worry about it.

I use them to mark rows in my garden and like FSM says they will take forever to break down. I suspect that there are numerous micro nutrients contained in the shells but probably ~99% calcium carbonate overall. Definitly a slow release amendment to any garden unless ground up to a fine powder first. Soil pH imbalance isn't a concern, esp with acid rain being the norm.

Oyster shells take about 20 years to decompose

Whole clam and mussel shells in the same class.

(except if you take the shells and run over them a few times with a 1 ton truck)

But Lobster/ shrimp/crab shells will give you some calcium the first year.