Will this help raise GH/KH?

I need to slightly raise the GH/KH for the intended species in my new 90L tank. I know there is calcium carbonate as well as crushed coral/oyster shells which I can get but I just found these from storage. I bought these originally for shrimp and my nerite. Would these work? And if they would, would a half of one or full one be ok?

They are unlikely to affect the GH or KH.

Shells (oyster and other types of shells) and dead coral rubble (crushed coral) is all made from calcium carbonate. It will raise the pH but won't increase the GH. It will slowly increase the KH, depending on how much powdered calcium carbonate you add.

If you want to raise the GH and KH of the water, use a Rift Lake water conditioner designed for African Rift lake cichlids. Depending on how high you want the GH and KH will determine how much you use.