I have a lot of bettas, all in seperate jars.
One of those jars always seems to have a funky, rotten-ish odor, even a day after the weekly water change.
I figured it was one of two things:
-that was the only betta tank with a snail in it, so possibly the snail had to do with it
-some malady of that particular fish
I have a feeling it is #2, as that fish never seemed particularly vibrant or lively, as compared to the other fish.
So... perhaps there is something inherintly wrong with one of the FISH, instead of your husbandry?
Just a theory.
I would also suggest thoroughly cleaning the gravel, or even removing it. And also use one of those scrubby sponge things to clean the glass and other surfaces in the tank (filter, decorations, etc.), in case the odor is connected to something adhering to those materials.