so they are ok to use.... how long will the dissolving take? they will only be in there for a maximum time of 2 weeks then i will take them out clean the tank and do a water change and put in me decor.
Over time, they will slowly dissolve. The harder your water, the more slowly they will dissolve. But if your water is more acidic, they will develop holes in them and dissolve rather quickly. I've seen this in a matter of months.
Calcium is readily available in things like bone, coral and shells. Drobbyb is right in that it will effect your hardness and pH and softer water will dissolve the shells rather on the quicker side of things. The more that's in your tank, the more drastically it'll effect your water. A stable pH and hardness is more important than an unusually high (but stable) or low pH, for example.