I understand what your using the medicine for. I was saying even if you remove your shrimp I would be hesitant about using the medicine, since the massive amounts of snails dying would cause an ammonia spike. Plus I was confused when you said fungus treatment, if its blue-green algae your after, than its a bacteria you are looking at.
Here is a little thread:
Hope it helps, as I've never had cyanobacteria.