I have just received a hung shen venus and that was potted with bits of oyster shell in the pot. I didn't know what it was at first and I had quite a scare but it is harmless enough. I will now go and ask my local fish monger for some spare oyster shells for the future.

I guess that if the plant has done well without it in the past, maybe it's better not to experiment.
I have added some slow release limestone to my cym mix this year, though. I'll see if it makes a difference.