Thanks scott,
I have some calcium in powder form which I would prefer to dose the tank with first. Just not sure how much powder to use. I don't want to raise my PH above 7 as my NW cichlids prefer it a little softer.
Thankyou for your input, I may well try the cuttlefish at a later date if required.
Mark
I'm afraid thats your problem then.
Nerite snails shells will begin to corrode below pH7, and once the protein layer on the outside has worn off, there's no repairing it.
If the water is only slightly acidic around pH 6.8 or something, you can often get way with it if you can get nerites with perfect shells, as the protein layer will protect the shell below from the acidic water, but if the shell is already showing white bits, it's only going to get worse for the snais.
Calcium is good for tanks with lots of snails, or if you have soft water (without much calcium in the first place) that's around neutral, but it wont help your snails in this case.
You need to raise the pH or move the snails to another tank, otherwise it will just corrode all the way through and expose the inside of the snail...not a good thing.
The fish are more adaptable than the snails, I would just add some calcium-based rock to the tank or some coral gravel to the filter.