Am i overfeeding my fish?

I used the tetra safe start stuff and my water has been nice for a while. But after a while it started getting a little murky and i did a 25% water change but not much changed. I went to pet smart to check my water and they said that it was fine. i feel that im overfeeding my fish. I feed them a lot of flakes because they always seem hungry and always finish all the flakes and are constantly looking at the bottom for more. I have 3 goldfish in a 10 gallon tank. I will be upgrading soon. Would overfeeding my fish cause some cloudiness in th water and if i make them fast would that help?