man cleaning fish tank

Five Common Reasons Why Your Tank Might Stink

One of the main reasons people get rid of or never purchase a fish tank is because of the odor. Before I go any further, let me tell you that my 36 gallon tank has never once emitted an odor. I keep my tank clean and avoid anything that could potentially make my water smell poorly.

Below are five common reasons why your aquarium may smell, along with some ways to fix the problems.

Improper or No Filtration

Most carbon filters that you purchase at pet stores only come with one filter. Filters should be cleaned every once in a while and replaced after a few months. Canister filters should be taken apart and cleaned every six months. If your fish tank has no filtration at all, then there should be no question of what is making your water cloudy and smelly.

When I purchased my girlfriend an aquarium for her birthday, I bought one of the 10 gallon kits from Amazon. This is a good starter tank, plus it came with a filter and full hood. When I was setting up the aquarium, I failed to notice that the included filter came with two cartridges, one white mesh-type insert and the carbon filter. Because the two inserts were individually wrapped, I assumed that one was a replacement filter and only installed the white mesh insert, leaving the carbon filter in the packaging… In just three days, some of the fish died and the water became very cloudy and foul smelling. I had no idea what was causing the smell, so we cleaned out her tank really well and filled it back up with water. In just a couple of days, the water was cloudy and smelly again. Come to find out, I didn’t even install the filter. This is called improper filtration.

Improper or No Cleaning

One of the most important aspects of aquarium care is cleaning the aquarium. For the best success, I recommend  doing a 10-20% water change every week, using a gravel vacuum very well once a month (draining about 30-40% of the water), and cleaning algae daily. If you never clean or maintain your aquarium, this is the reason it smells. Even with fish that help clean things up, you still need to take responsibility and clean your tank.


No matter what the pet store salesman may tell you, don’t overpopulate your fish tank. The general rule is one inch of fish per gallon of water, but remember to keep in mind the size the fish will be when it’s full grown. If your fish tank is only 10 gallons, that means you can get 10 fish that will be 1 inch when fully grown, 5 fish that will be 2 inches, or you can mix it up. Just remember that although fish like Rainbow Sharks are purchased when they are about 2 inches long, they can get up to 6 inches after living for just one year.

Overpopulating your fish tank leads to more fish pooping, which leads to more waste on the bottom of the tank. Plecos don’t eat fish waste – that’s what a gravel vacuum is for. If you overpopulate your tank, it is bound to smell.


If you look at your water and notice that it has a green hue to it, this means that you have algae growing. Algae can be reversed by buying chemicals from your local pet store and buying algae-eating fish. Algae is caused by improper filtration and excess sunlight. If your tank is near a window and gets more than six hours of direct sunlight a day, then you are going to get algae.

Dead Critters

If you have made it this far on the list and none of the above reasons apply to your tank, then you could have dead organisms in your tank. Dead fish (easily spotted), snails (not so much), and other critters give off a potent odor and make the water cloudy. I would suggest doing a good sweep with the gravel vacuum just to make sure you don’t have anything dead under the substrate. If you are having a snail infestation, I would suggest trying to get rid of them.


If you still haven’t found why your fish tank stinks, I would suggest asking aquarium forums or testing your water with a water testing kit. There are numerous other reasons that your aquarium could smell bad, so don’t think that this is it. And remember, if all else fails, buy a new filter, drain out 100% of your water, and start fresh.

Scroll to Top