Feeding snails.

they are a freshwater snail, so i don't even know if they would breed in brackish. if you put them in there, your puffer might nip at them enough to damage them even if they don't die. i would recommend getting either a large bowl with a small filter or a small tank set-up and putting them in there. a bunch of floating or anchored plants (any aquatic or plastic kind, just throw them in there) will give them places to attach the eggs. i've noticed that after i throw in a large amount of food (like leftover veggies from dinner) they will lay eggs. they don't need a heater or even a lot of water changes, and it's fine to have the tank bare on the bottom. filtration is important because if they dont have it, they'll just sit at the waterline and never really move around.
if you need more snails, i have some for sale in the classified ad section.