Revisiting this thread. I keep getting this smell from my tank and I don't know why. It goes and comes back. I thought it was being caused by the canopy I had on the tank so I got rid of the canopy. I did a large water change on Sunday and the tank smelled yesterday. I took the filters apart and cleaned them thoroughly but they were not even that dirty. The only thing that was dirty really was the purigen which I essentially tossed. Could purigen that has been in the tank for a long period of time be the cause of the smell?
There is no food anywhere, I stirred up the sand again, and I do weekly 30% water changes which is pretty good considering it is a 180g with 1 10in fish that eats only 1-2 times a week. The only other thing I can think of is that I fed my fish on Monday, but not seeing any food lying around in the tank I dont see how that could be the cause.