i know for a fact that petsmart doesn't feed their snails the necessary calcium enriched diet. perhaps try posting a wanted ad in the classifieds here if you want to try again with these snails. if i was still breeding them i'd send you a few!
this was one of my babies, a beautiful female jade
a 2.5 gallon isn't large enough for an apple snail and a betta. these snails do make a good amount of waste when cared for properly and you'd be doing nearly daily water changes on a 2.5 to keep it clean enough for the snail and the fish.
if you do try again, here's a great recipe for snail jello:
1 jar baby food (i used the butternut squash variety, my snails loved it!)
about 10 Tums, crushed up into fine powder. flavour can be any, but i used the fruit ones. this is your calcium source.
1.5 packets knox gelatin (recipes i found online say only 1 packet, but i found my snail jello too runny with just 1)
crushed up fish food, if desired. about 2 tablespoons. (i used nls pellets, my snails were spoiled, lol, but flake or any other dry food would work)
nuke the baby food in a bowl in the microwave for a minute on high. stir in the knox gelatin, ensuring there are no lumps. stir in the crushed up Tums and fish food. make sure it is mixed well. spoon into an ice cube tray, filling each about 1/2 full. put tray in freezer until the cubes are set. remove cubes from tray (you'll need to dig them out with a spoon) and store in a ziploc in the freezer. they'll stay good a long time.
i found that a cube the size i describe above is about enough for 4 or 5 snails each, and i would feed the snail jello a couple times a week. if you only have a couple snails, then either make smaller cubes or cut the size i mention in half.