It may be from accumulated wasteproducts from gravel / substrate. Perhaps removing the fish to another container while you thoroughly stir things up to have removed by a filter and/or water change?

I also suspect the substrate, along with too few and small of water changes. In the closed system of a tank, regardless of how well you filter, the fish are still swimming in their own sewage, and sewers- stink.

With a good biological system, and for that, any in-tank foam sized to the tank will do the job, you can change 50% every week and not lose the bio, while still mechanically filtering out the junk.

when you do the water changes are you vacuuming the rocks ? can't help but think your filtration system is not doing its job. May be time for a new filter.

IMO a 50% water change every week is too much. You've got other problems that need to be addressed if the water smells that bad.

I have too many big gold fish in a 75 gallon aquarium, change the water once a month and it never smells.