Had-a-snail & Valisneria

Karen Randall wrote:

>  If your plants really are a Valisneria sp., or as is more likely, a Sagitaria
> sp. a copper based medication is definitely a bad idea.  Both genus' are very
> sensitive to copper.

I have noticed that Had-a-snail is also copper-sulphate. Is it also true
that this product at the recommended dosage would damage Valisneria?
It would seem that the yellow spots/holes that I observed may not be caused
by the snails but by a shortage of Potassium (presumably it was all used up
by the rapid growth) and that they are in fact beneficial algae eaters. 
Curious if anybody else has any opinions pro or con snails. Probably most 
people avoid them for aesthetic reasons. I used to have a couple of large 
ones that were quite interesting to watch.

 - Steve