What does it smell like?
Do the surface bubbles pop after 15 seconds or before?
Does the surface look oiley or gross?
Do you add water streight from the faucet or let it sit in a bucket 3 days with an air stone, after adding a dose of Amquil plus.

If your gravel is black underneith it is making a gas that smells like rotten eggs.

The following link is good information about water.
Aquarium Water Quality.