Smelly fish tank issue

[Blacklight]

I'm having a problem where my 5 gallon tank is stinking like a swamp and it's beginning to get fairly strong despite constant water changes and gravel vacuuming which we do once every 5-7 days.
The amonia level is at 0. The PH is at 7.0.
The filter cartrige looks fine and is only 3 weeks old.
We don't feed flakes, just pellets (and those are in the water for less than a few secconds given at how quick the betta eats them) so there shouldn't be any decomposing food in there.
I always make sure that I give the gravel a good vacuuming when I change the water.
Yet despite all this, the tank is getting a very strong swamp smell to the point that it's beginning to stink up the bedroom where it's located.
I've had other fish tanks in the past and none of them ever made an odor like this.

Anyone have any idea as to what's going on and how to remedy the problem?

[ChoclateBetta]

How many water changes are there decorations?

[Blacklight]

Yes. There's decorations. We do about a 50% water change each time. We change water every 5-7 days.

[ChoclateBetta]

I think more water changes like double and maybe a 100 percent will help.

[LizzyP]

If you're cycling the tank, never do 100% changes.

Do you have any live plants? If not, really stir up that gravel with your vacuum. A lot of poop and crud falls between the stones and gets stuck underneath the gravel. Try doing more water changes per week. I would up it to 2 50% changes per week.

[ChoclateBetta]

Makes sense in my 10 I do 1 50 so 2 50 percents for you is good addibg pothos to the filter will help.

[Friendlyfishies]

My 20gallon long smells the same way! Its been pretty bad since I added the driftwood and hygrophila plants, yuck...I dont even have fishes in there yet!

[ChoclateBetta]

I wonder where the ammonia came from?

[Blacklight]

I have a theory that when my wife did the last water change that she didn't do a good vacuum of the gravel. Tomorrow, I'm going to get out the siphon and force her to watch me to see how it's supposed to be done.
In my years of aquarium ownership in the past, I never smelled a tank this bad. And this tank has ONE fish. My old tank was a 10 gallon and had a freakin' crowd of fish and it never smelled.

[Wendyjo]

Originally Posted by ChoclateBetta

I wonder where the ammonia came from?

What ammonia?