Wow, very nice tank. The CO2 will kill the snails and slugs. True it won't kill the eggs, that's why the CO2 treatment will need to be repeated. It will work trust me, it did for me. Don't skimp on dry ice, make sure you overflow the tank with it, then top it off an hr later...You'll need to move the frogs into another tank of course
I wouldn't trust any substances with frogs around, they're just too delicate