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I actually picked up this idea back when I had puffer fish.

Snails are a great occasional food for a turtle.
She'll contains a lot of calcium
Live snails provide good nutrition
The chewing the shell is good for the turtles beak
Again don't over feed as anything with a lot of protein

If you go to any pet store , you'll notice many of their aquariums have the small snails that arnt for sale they generally come from live plants. These snails breed very fast and are generally just a pain in the butt for pet stores.
I personally go to pet smart and ask them to sell me some and more often then not they go talk to their boss and the boss say just bag them up and give them to me for free. The odd occasion they will charge me 10c.
There's 2 basic types of these snails the more oval or round shaped snail and then the unicorn horn shaped ones.
Ideally you want the round or oval shaped ones as a turtle can crush their shells no problem. The unicorn shaped ones have very tough shells and you could use them but your turtle would prob have to swallow whole.

If you make trips to the pet store often then you cN pick up 10 here and there as a turtle snack.

If not, breeding these snails is very very easy. Get a 1-2 gallon jug tank anything. Fill with aquarium water and a little bit of substrate. Crush turtle food to feed the snails and within a week or 2 you'll probably see some babies crawling around.
Every couple of days remove like 50% of the water and replace wih new quarium water. The snails lay eggs that stick to the a surface so you won't remove the eggs when changing out some water

I think the ones you are recommending are mini ramshorn snails

http://aqualandpetsplus.com/Bug,%20Snai ... shorns.htm

And the "unicorn" shaped ones are likely malaysian trumpet snails

http://www.planetinverts.com/malaysian_ ... snail.html

Though, there are lots of possible snails it could be. I would avoid those because, like you say, the shell cannot be crushed. And the larger ones run the potential of an intestinal blockage.

Nah they arnt ramhorns, ramhorms have a tough shell due to the structure.

Physella acuta
http://en.wikipedia.org/wiki/Physella_a ... ta_001.JPG

These are the snails you should be looking for.
They have very weak shells and stay around 4 millimeters and breed like crazy.

Setup my snail tank 4 days ago and I already see about 5 clusters of lie 25-50 eggs

In a separate tank, I "grow" ramshorns, then when Thor wants a little es-cargo I pick out two smash 'em into snail pate' and Thor licks his chops. The point here being is that you don't need to feed whole snails to your RES, whole, they get the same benefits if the snails are smashed up

btw, Travis is correct about the (what we call) "pond" snails. they do reproduce very fast, so much so, that I'd have to feed Thor pond snail pate' at least once a week just to keep the snails' population manageable.