How often should you change your water? I have a 30 gal that I have been changing 5 gal every 2-3 days. I was in PetSmart and told the lady that (was talking about the clarity of my water) and she freaked out on me saying I was throwing away all my good bacteria. From what I've seen and read, this is kept on the items IN my tank and not on the water itself. Is this correct or was she right?