The most commonly used rule for tipping here is: Between 5 and 10 percent.
I think that it's necessary to provide some background to the prices as well: Most restaurants have large profits on drinks, not only on meals, therefore you quite often pay even for plain water, because they don't want to lose their profits. However, no tips are included in the prices (this may differ if you pay by card). Sometimes, there's an extra payment for service, but this is mostly in places you don't want to visit for a generally bad price/quality ratio.
Can you give no tips at all? Yes, of course, especially if you weren't satisfied.
Can you give larger tips than 10 %? Yes of course, especially if they were willing to fulfill your special wishes, were really friendly etc.
It doesn't hurt to give some feedback, especially if something was extremly good or bad. As always.