Also, I do notice that 2 apple snail, the shell is getting thin and soft. What should I do? If not, they'll surely die.....

Thanks for your advice and much appreciated!natefrog wrote:Thin shells in mollusks is a sure sign of a calcium deficency. If your water is fairly soft and acidic a snail can lose more calcium to the water than it can replace. There are two ways you can help this problem.
One is to add crushed coral or shells to the tank, this however, will increase the hardness of your water and will act as a ph buffer and make it difficult to maintain a ph below 7.
Option 2 is to feed your snail with something that is calcium rich. Off the top of my head I do not know of which vegetables are highest in calcium but I'm sure with a little research you can find out.
I would likley go for option 2, but that is because most of my tanks are South American and therefore enjoy a ph of below 7. You could try a bit of both if your tanks can handle a slightly higher ph and a higher dissolved hardness.
Good luck.