I have had a rabbit snail in my tank for a few months now. I hadn't given much thought to my pH prior to this and he was always fine. Recently I noticed his shell was deteriorating, and it seemed to coincide with a ramshorn snail explosion in my tank and they too were looking ratty, so I figured they had taken all of the calcium out of the water. I added an eggshell to the tank and smooshed as many ramshorns as I could reach (leaving their shells in the tank). That didn't help, in fact he got worse; the eggshell didn't seem to dissolve into the tank at all. I have since added 2-3 eggs worth of very finely crushed shell to my tank and filter. I didn't test my pH before adding the eggshell but the tank has always tested around 6.5-7. My pH is hovering around 7 and I don't want to increase it because everything else in the tank is doing great. It also doesn't seem low enough that it should be affecting him. So far the snail doesn't seem to be getting better, and he is not very active.
Is it possible that the eggshell is taking a while to "kick in"? It has been about 5 days now since I have been adding the crushed eggshell.
You can try cuttle bone for a source of calcium but you will have to find some way to keep it submersed. I use one of those veggie clips. You can get them from PetsSmart and Bulk Barn.
I seem to remember someone with the same problem.
It seems CO2 causes this. Not sure if adding calcium will help.
http://www.plantedtank.net/forums/88-shrimp-other-invertebrates/165692-snails-co2.html (http://www.plantedtank.net/forums/88-shrimp-other-invertebrates/165692-snails-co2.html)
Thanks - I don't inject CO2 although I do dose with excel. I think I'll give this guy away. I don't want to hurt him and I also don't want the hassle of playing with my pH / calcium constantly.