If you are using a simple kitchen sieve, you are probably crowding your fry pretty badly. A larger enclosure like a colander might work out fine though. Even a mere 6 fry will need some swimming room to truly thrive. I try to use a minimum of a 5 gallon, 18 litre, tank as a grow out setting for even the smallest of my fry. For mollies, that space would start to be too small after only about a month.
Caring for fry also means lots of water changes to remove any uneaten food, even the bits too small to see easily.