How to raise calcium levels?

i ordered some of the mini/,small calcium chips from crayfish empire because i just got two mystery snails and one of them has some chips to the bottom of his shell and it loooks thin. i was wondering what should my levels be and how do i add/use the calcium to raise the gh and kh of my 2.5 gallon tank without harming my betta or snails.

 

You can add cuttlebone

 

Also (sterilized) eggshells

 

A bit of crushed coral works really well. It will add a steady supply of minerals to your water. If your tank is on the acidic side, it will dissolve slowly. I have just a few pieces in my planted tank to keep the snails happy.
What else is in the tank?

 

Eggshells are easiest just wash them with water then put them in the oven at 150 C for 20 minutes.