When you get the new fish ... the person at the pet store scoops water into the bag. That water must have had some bonus features.
I'd swing back to the store and see what type of snails they have. Most likely, they'll be beneficial to the environment ... possibly less cleaning required.
Then again, they might be mutant killer snails that can grow to 1000X their current size in a single evening.... you never know.
Both thoughts have equal weight in my mind. Although on saying that we haven't been to the fish shop since last year, I was just thinking how well the current crop were doing. All I have done lately is change the filter.
There mus have been some cross contamination somewhere. I have my fingers crossed for beneficial and tank cleaning. Rather than gigantinator man eating things!