[Cavan Allen]

In severe cases you can repair their shells with reef putty. I never did get to the bottom of the causes myself (although it never happened in low-tech, non-co2 tanks), but that really does work, and I kept some alive for years that way. Not pretty though...

 

[Cavan Allen]

Just pull off a piece, mix the two components together, put some on the shell where it's eroded and plop them back in. It hardens under water.