Overpopulation of snails generally indicate abundant food sources, ie. overfeeding is the most common cause. Now with predators, it's a slightly different story since they are such messy eaters it leaves plenty of leftovers for the snails. You may need to look into a better cleanup crew.
Most snail killers are copper based so I would assume in small quantities that it would be ok for your fish. They are harmful to inverts such as freshwater shrimp.
Have you considered other types of botias?
There are also 'Assasin Snails' that are a slower method of controlling a snail population as they are carnivours. Basically, they eat other types of snails aside from their own. You're probably thinking, "Why would I buy new snails to get rid of the old ones?" It's because they reproduce at a very slow rate so it would be very hard to be overcome with them plus, they look cooler than regular snails

. Or give them away to other people that have the same problem

.
Have you ever tried putting vegetables into a container and dumping the snails once they're all gathered on it? It may take a couple attempts but it could help.
Hope that helps!