Feeding foods high in calcium will help. I make snail jello (the original recipe came from the boards on the site you found) and my snails LOOOOVE it. I'm getting nice strong growth on them and their shells are not eroding at all. You can also feed canned turnip or mustard greens, softened (freezing works well) kale, collards, mustard or turnip greens. All of those have good calcium content.
My snail jello recipe is:
1 jar baby food (look for the highest calcium content...green beans are a good choice)
1 envelope plain gelatin (knox is a common brand)
1 tsp pulverized shrimp pellets or pulverized hermit crab cakes
1 tsp calcium powder (Jurrasical is the brand I use...for dusting crickets to feed to reptiles)
a blob or two of canned greens
Heat the baby food in a micro safe dish for 45 to 60 seconds...it needs to be VERY hot. Stir in the gelatin powder until it is smooth, then stir in the pulverized stuff, greens and calcium. Pour it into a flat dish or onto plastic wrap (let it cool a bit if you use the wrap...I melted some once LOL!). Pop it in the fridge until firm and cut into pieces. Store in a zip top bag in the freezer and break off a few bits when you are ready to feed. It will thaw in your hand in just a minute or two.
For the fissure, go light on the feeding though. Also, if you can drop the temp in your tank a bit it will help as well. You want to slow the new growth so the snail can repair itself. Are the other fish picking on your snail? With the exposed flesh, he may need to be isolated until he heals.
I suspect your pH and kH are a bit on the acidic and soft side. Consider using crushed coral in your filter to buffer that up if it is low. Snails do need a higher pH and kH. I strive for at least 7.6 pH, but don't bother with that pH up stuff. It will just cause swings and problems.
A lot of people on the forums at applesnail.net dose calcium, but I'm not as of now. The dosage varies by who is dosing, but it seems 1 drop per gallon per day is pretty widely accepted. That dosage is for the Kent's brand. Seachem makes one, but the dosage is different I think and I can't recall it right now.
