I have a 55 gl tank established bio filter set for a 100 gals areator 5 fish I clean regularly and add fresh water but the water smell stale or stagnent all levels are fine?

What could it be?

C2

How long has the tank been set up? What exactly is in it? How much water do you change and how often? If it's well aerated and filtered, it shouldn't stink (a light fishy smell is all). I suggest doing a 50% water change and putting fresh carbon in the filter. Carbon is good at removing stinky compounds.

C2, how's the water doing now? Any change in the odor?