I have a 2.5 gallon, and I do a 50% water change on Wednesdays and a 100% on Sundays. I don't leave old food in the tank, I don't overfeed (I think), and the only live things in the tank are my betta and a moss ball.
My tank water smells like rotten eggs, and I would assume that it's my tap water, but water straight from my tap doesn't smell at all. Jalen isn't acting sick or anything, and he doesn't have a nose, so I'm guessing that the water isn't bothering him. The smell just makes water changes unpleasant, since the stench has a habit of wafting up when I pour the old water down the drain.... *gag*