There are hundreds of reasons a tank could have an odor. Be specific
Without a description of the odor (as someone else posted) its difficult to figure.
Rotton eggs? = Hydrogen sulfide building up in the substrate, from not enough vacuuming.
Fishy? = Dead fish or rotting food.
Other rotting smell?....Plant debris rotting
Earthy?.....All tanks have a natural earthy smell.
Something else all together?
Is it a smell only your wife or a woman can smell.
My "X-wife" could smell my basement tanks, up-stairs, I wouldn't notice the smell right next to them.