Aquarium snails...

Your goldfish are consuming the snails. They love them. I know this from experience. I had a tank of goldfish and added snails to the tank. Less than a week later I noticed that the snail shells were on the bottom w/o snails.

I honestly don't know if goldfish eat snails or not, but it is possible that they do. But my first guess is that your water quality may be suffering. Snails can be sensitive to poor water quality or chemicals in the water. You said that your tank is only 10 liters, which is very small. Even with regular cleanings, it is very difficult to keep stable water quality in such a small amount of water. In addition, 2 goldfish is probably all that the system can handle. In fact, if they are large goldfish, it would qualify as "overcrowded," meaning that your fish produce more waste than the tank system can adequately handle. I would not recommend adding any more creatures to your current aquarium. In addition, you may want to have your water quality tested. Many pet stores will do this for you if you bring in a water sample. Or you could buy a water testing kit, which is a good thing for any hobby aquariast to have around.