Smell???

Freshwater or saltwater? What kind of smell?

right, tanks should have a slight earthy smell, but you can only smell it if you put your nose right over the tank. If it's an overbearing noticeable stink, something isn't right. It could be too much ammonia, or possibly sulfur from anaerobic bacteria.

What is your water change schedule? Clean water, maintained by at least 25-50% water changes a week, should hardly smell at all. Do you gravel vacuum when you do water changes?

So an aquarium isn't a self-contained ecosystem. It requires regular physical removal of wastes, otherwise they have nowhere to go and just build up.

Of course if you spill a lot of water on the floor and live in a humid climate you could have mold/mildew growing on your carpet.

I've found the best way to reduce tank smell is:

1. Increase water changes so that nitrates are really low (say 0-5 ppm)
2. Put carbon in the filter (temporarily until smell is gone if there are live plants)
3. Reduce the amount of gravel to a thin layer (if the tank is not planted) so that there are fewer places for food and waste to hide and decay.
4. Gravel vacuum at least once a week, twice if necessary.
4. Remove all rotting plant matter and algae build up on plants, driftwood, and gravel.