I know they eat the algae but what else do they eat? Do they just eat what ever is in the tank? do they eat flake food?

Most snails will eat flakes or most any kind of fish food and some unfortunately enjoy aquarium plants as well. Nerite snails are pretty much algae only although mine do eat some spinach. But ramshorns and mystery snails will eat about anything. Shirmp pellets and algae wafers they both seem to like.

And some snails eat snails... Assasin Snails will rid your tank of unwanted snails that often appear out of nowhere.
Alot of the common snails bought, mystery, apple, horn, as mentioned will eat plants but only if really hungry, they actually do a good job at eating dead plant matter first.
Assasin after ridding the tank of snails can be feed other things like beefheart, shrimp chunks, pellets.
Were you thinking about a particular snail?

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"Buddha in your tank"

If it's for your Saltwater Tank mate I wouldn't worry about feeding them, they will eat the Algae and there will be plenty of stuff here and there all over the tank for them to eat.

They are like the Hermit Crabs you buy, they eat whatever they find.

What kind of snails are you thinking about? If you have a mystery snail or apple snail (the big ones with shells about the diameter of a quarter, sometimes bigger, smaller if young) feeding is important, since the most common problem in your average aquarium is starvation.

i believe i have turbo snails. i looked up photos and they look just like that...what about my sea hare? u guys know what he eats once the algae is gone becasue the snails and the hare are going to town on my aquarium(their basically licking their fingers when they are done)lol.

The snails don't worry about feeding they will find something kicking about, you will notice every now and again a snail may die while the rest seem fine, nothing much you can do there except buy some more, my own opinion and this hasn't been proven as far as I'm aware is that because we build/design your tank to be like the sea but Snails and Crabs don't really come from there they are gathered up in pools on the beach or shallow water, again that's my own opinion or guess may be a more fitting assumption.

As for the Sea Hare when the algae is running out the best thing to do is to take it back to a LFS and trade it in that way another person with an ALgae problem can pick it up and it's life cycle will carry on, when it runs out of Algae it will more than likely just starve to death then die when this happens they will release poisons into the water and none of your other Clean up Crew will go near it.

Try and get a hold of some Mexican Turbo Snail they are good eaters and will eat Hair Algae even the red stuff which a load of snails won't, I have some Cerith Snails they burrow under the sand which is good as they will turn it over and you can't go wrong with Nassarius snails either they also like to burrow under the sandbed.

Well first I gotta say DOH!... Cause I was advising on fresh water...
In response to Verdifer, some snails do just seem to die, but snails often seem dead and are tucked up for a very long time but are not dead. So don't be to quick to toss them, when my reef tank was up there was many snail shells. When they were really dead they relax their "door" enough and my fish picked them clean.
In a good running tank you shouldn't have to worry about them starving, same with the Hare's. You could buy some Chaeto and put it in your refugium (if you have one set up) or in your filter, and use it to feed the Hare. Or introduce any other macroalgae in a similar manner. In my SW hospital tank I would keep some rock cultivated with algae growth long hair and green (always some inevetible brown)on it to rotate into the main tank. And of course there are products you cam buy that will be fed upon as well.
And the types of snails Verdifer mentioned are really good if your doing a "Jaubert" sand filter.

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"Buddha in your tank"

thanks guys.