My betta (in a 10 npt) attacks and kills snails regardless of size, both ramshorns and pond snails. It's actually quite entertaining, though I wish he'd leave them alone sometimes so they could eat leftovers et cetera.
I've had similar symptoms that you described in the 125 with ramshorns after pouring H2O2 at relatively high rates to the point that I only have a few individuals left of a colony of 100+ (not counting pond snails!!!-they've ALL gone away!). This wasn't a bad thing in my mind, but demonstrates that snails are pretty sensitive to oxidizing agents
...just something to consider while you try to find the cause of the issue.
Then again, your frog's health is going to be a really good indicator of water conditions as well. If he's not acting strange you'd think that all was well regarding toxins, especially metals. Perhaps he is exuding something the snails don't like??? I guess it begs the question: What type of frog do you have?
Anyway, if you get the answer you're after let us know so we can avoid it. Nerites may not be really expensive, but they certainly don't replenish themselves.