Combating foul smelling tanks

Try some charcoal in your filter. That has worked for me in the past. Just try to change it every 2-3 weeks cause it loses it's effectiveness quickly.
Len

i've heard of people saying their tank stinks, how does it happen?

i've been having 2 tanks in the hall for more than a year now, that doesn't happen.

now with the added carbon on the filter, it's not likely to happen at all.

I've never had a problem with my tanks smelling. I used to think it was because I used carbon...but I haven't had carbon on my tanks in months and there is no smell.

JamesBenjamin,

I'm sorry to tell you this, but a foul-smelling tank is not a normal thing and I would look to the frequency/intensity of your tank maintenance as the cause, excluding the possibility that there may be a dead fish in there, that is.

Being in the medical field, I can tell you that a foul smell of any kind is linked to an over-growth of bacteria. Not to get into too much detail, but the foul smell itself is a by-product of the bacteria themselves. In the case of your aquarium, we could be talking about dead fish, overfeeding (i.e., rotting food), an accumulation of wastein the substrate, or dirty filter media.

First and foremost, make sure all your fish are accounted for. Then, not only do a major water change, but vacuum the daylights out of your substrate, taking care to do the same to your decorations. Check your filter media and make sure it does not have an over-accumulation of waste products. You may need to do these things once or twice more until you notice a change in the odor.

A foul-smelling odor is a symptom of something and while carbon can eliminate the "odor", it will not eliminate the "cause".

I crossposted with TnCGal's excellent post, so I'll just add now:

And more oxygen. Oxidizing skunky metabolic by-products like thiols ("mercaptans") oxidizes the sulfur into sulfate and renders it odorless. Get some splash from the filter return. Open the top glass a little more. Even 5 ccs of hydrogen peroxide (3% solution from the drugstore) per 10 gallons, slowly added to the filter outlet, can help oxidize organics.

And better housecleaning. Are you using a canister filter? Are the filters backwashed often enough? Carbon is a good idea.

The aquarium shouldn't smell, not more than a faint "green" smell-- in a planted tank.

Did you test your nitrates? if so, how much is it?

More oxygen and water turbulence! What filter do you have on the tank?

Oftentimes smelly water can be fixed by just adding an airstone and doing more frequent water changes.