You should keep your sink clean and dry after using it to avoid those insects inffestation. I'm not sure it's fleas - they just can't fly. Fruit flies and other little house flies can be killed with Zyrox granules. It's very effective, but you have to remember that it's toxic, so if you have pets they shouldn't touch this stuf. Here is the article where one can find the description of other good remedies against flies.
