I hope you got them all then! Just to make sure, take it out during the day and put it back in overnight for a day or two.
If I'm not mistaken, I believe Canas are illegal in Australia? I'll have to check on that but I remember reading that Australia has insane (but understandable) rules on snails, especially the plant-eating ones like Canas who can get into crops.
As for the food issue, check that site I posted. They love most green veggies, but some are possible calcium "blockers" that prevent the absorption of calcium, which is critical to shell growth and maintenance. Some of these "bad" foods are broccoli, cauliflower, and spinach. They'll do fine on the diet you're feeding them though. If you're not adding a calcium supplement to your tank (Kent Marine Liquid Calcium, cuttlebone, crushed coral as a substrate, or even crushed eggshells) then try feeding them calcium-rich foods such as kale as well.