I've always used a cuttlefish in my GAL's tank but with the moisture they tend to dissolve quite quickly.

Is there an easy way to prevent this or are there any alternative sources of calcium that I can use? I'm not too fond of using calicium powder as I know that too much calcium can be harmful.

Cutting/snapping up the cuttlefish and giving it a bit at a time will make it last longer. Or putting it in a dish or finding a way of hanging it up at the side of their tank maybe, so it's not constantly on damp soil?
Feeding calcium rich veggies are always a good option too, spring greens and squash are great for a calci boost.

Oh that's heplful, thanks!