thanks for those posts, they are both helpful.
I did not think i was over stocked, i have tested the tap water after
a week and it is stable, i dont think i am over feeding.
Decalcification sounds like a good straw to cluch at, could the
process of the snails growing their shell and then getting removed
from the tank by me be using up calcium too?
I think i will try the coral - or at least read up on the subject!

chris