About snails...

What do you mean by the tips of their shells? Do you mean at the ends of the spires (the pointy swirly bit), or around the mouth of the shell where the snail's body comes out?

Soft water is very bad for snails. It will make it difficult for the snails to gather the calcium from their environment to keep their shells hard and healthy, which leads to decay of their spires (which may be happening to your guys), and leaves their shells prone to cracking. Things you can do to try and rectify the situation include using a liquid calcium supplement for aquairums, adding crushed coral in a media bag to the filter, feeding high calcium veggies, and dropping a cuttlebone in the tank for the snails to graze on.

In terms of liquid calcium supplements, most people recommend using one made by Kent, and I think it is used at a lower dosage than written on the bottle. I'm not overly familiar with this method, since I can't find the product in question around here anywhere, but many people swear by it.

Adding crushed coral to your filter is also effective, but be careful, since it will alter the chemistry of your water a bit. Namely, you can expect your pH to raise up a bit. Depending on your other tankmates, this may or may not be a problem, but I would think adding the coral in small amounts would help acclimate everyone to the change over time and keep your tank residents happy.

High calcium veggies that snails go ape-poopy over include spinach, dandelion leaves, and kale. There are of course many other options, but these are just some that popped into my head at the moment. By the way, your snails will appreciate just about any veggie you add to the tank-- carrots, romaine lettuce, potatoes, apple slices, cucumbers, zucchini... Just make sure the veggies are washed properly, first. I like to pull out a nice assortment of goodies for my guys every time I make a salad. Sometimes blanching vegetables first will help soften them up and make them sink faster so its easier for your snails to eat them, but I've found that my snails don't really care if their dinner is blanched or not. They're gluttons!

Cuttlebones are those white ovaly shaped things you can find in the bird care department of your local pet store. It will float when you first drop it into the tank, but you can weight it down by wedging it under a piece of driftwood or something. Eventually, it will stay down on its own and you can move it to a less conspicuous place. Some people's snails actively graze on the soft side of the cuttlebone, so leave that side up. Othertimes, the cuttlebone is left alone and will simply leach some nutirents into the water-- if this is what's going on in your tank, make sure you keep checking on the cuttlebone regularly to make sure nothing icky is growing on it.

As for the ich possibility, I'm not sure if snails can get it or not. I do know that some of my snails have spots on their bodies that are just part of a cool colouration pattern they have naturally. However, my snails are brigs-- I don't know if trumpet snails have a similar kind of markation, but I wouldn't be surprised.

Well put Mandi! I would like to add that with your cuttle bone, if you boil it for about 30 minutes (give or take a little depending on the size) then it will sink from the get go so you can hide it. I personally break a large cuttle bone into three parts as even for my snails that are bigger than golf balls, they don't eat it all.
Get some Kent Calcium (found in the reef section of you LFS) and you can either add half dose as recommended by many or even full dose. Someone had told me that they read where someone tested this at full level so I gave it a shot to see what would happen. Well shells are strong and looking good. Fish don't seem to be bothered at the calcium except for the fact that they think I should feed them when adding it to the water.
Snails cannot get ich so don't worry about that.
A good diet will help your snails grow and build their shells. I'll post a list of foods here in a second.
Keep in mind that with providing calcium and proper pH (nothing below 7.0, however 7.5 is much better and even higher is better), your snails will build strong healthy shells though what damage to the shell is done and will be there for the snail's life.
As for crushed coral....A must have. It will maintain your pH at approx. 7.5-7.8 It will take a long while for the pH to ease up to that level (took me two months) but that's actually good so that any fish have a chance to slowly accommodate. None of my fish (even those that like the lower pH range) have had any problems adjusting but, as with most/all fish, if the pH is not in the right range, they won't spawn. I personally have no problem with this seeing that the only pairs I have are platties (whom are still spawning left and right) which are just allowed to breed to feed my angel and peacock eel.
Now, for that list of food:

High Calcium foods for snails:

Calcium per 100g serving

Veggies:

Dill Weed 208mg
Turnip Greens 190mg
Collards 145mg
Parsley 138mg
Kale 135mg
Watercress 120mg
Beet Greens 119mg
Chinese Cabbage 105mg
Mustard Greens 103mg
Chicory Greens 100mg
Spinach 99mg
Okra 81mg
Leaf Lettuce 68mg
Cilantro 67mg
Purslane 65mg
Endive 52mg
Swiss Chard 51mg
Broccoli 48mg
Cabbage 47mg
Rutabaga 47mg
Crussel Sprouts 42mg
Celery 40mg
Sweet Potato Leaves 37mg
Green Beans *Tend to only like French Cut* 37mg
Romaine Lettuce 36mg
Parsnips 36mg
Head Lettuce 32mg
Alfalfa Sprouts 32mg
Winter Squash *all varieties* 31mg
Turnip 30mg
Carrots 27mg
Kohlrabi 24mg
Sweet Potato 22mg
Cauliflower 22mg
Asparagus 21mg
Pumpkin 21mg
Summer Squash *all varieties* 20mg
Beets 16mg
Cucumber w/skin 14mg
Red and Green Peppers 9mg
Tomato 5mg
White Corn 2mg

Fruit:

Seedless Rasins 49mg
Orange 40mg
Lime 33mg
Blackberries 32mg
Kiwi 26mg
Lemon w/out peel 26mg
Papaya 24mg
Raspberries 22mg
Sweet Cherries 15mg
Strawberries 14mg
Tangerine 14mg
Apricots 14mg
White Grapefruit 12mg
Red & Pink Grapefruit 11mg
Pear 11mg
Cantaloupe 11mg
Grapes 11mg
Mango 10mg
Watermelon 8mg
Persimmon, Japanese 8mg
Pineapple 7mg
Apple w/skin 7mg
Cranberries 7mg
Banana 6mg
Honeydew Melon 6mg
Blueberries 6mg
Casaba Melon 5mg
Nectarine 5mg
Peach 5mg
Plum 4mg

*Resource from www.applesnail.net*

The white on your snails shells was not caused by you. It develops over time. All the advice that has been given to you so far in this post has been dead on!

With MTS it is VERY common for the tips of the shells to turn white and break off. It normally will not kill the snails unless it is an extreme case. Most of the time they will live the rest of their lives happily but without a point at the end of their shell.

And just to verify, yes Kents can be used at full dosage. I often dose mine at higher then full dose because my tanks are more heavily stocked. When dosing be sure to add the calcium into moving water like where an airstone is or where your filter dumps into the tank. Do not apply the calcium directly to your snails or fish.

Interesting. What does the addition of fresh fruit and vegetables do to the water condition of the tank?

As long as they are left in too long ( I just leave mine in over night) nothing.

I would be leary of adding fruit to a tank. I very often have veggies in my tanks for 24 hours before they all get eaten and never have any water problems.

What is the diff in adding fruit vs veggies???? I was thinking of adding a grape.