Aquarium smells

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Most aquariums are relatively odorless. However, sometimes aquarium owners notice unusually smelly tank water, and that shouldn't be happening. Basically, if you can smell your aquarium with the lid closed and you are in another room, there is an issue that requires your immediate attention.

In this guide, we're going to take a closer look at (or sniff!) Aquarium odors to find out what is causing the pong and how to fix and prevent those unpleasant odors!

Does a healthy aquarium smell?

Yes! A healthy aquarium has a light aroma. The smell is not fishy or foul. Most hobbyists actually describe it as quite enjoyable.

Fresh water tanks have a light, earthy smell, similar to freshly plowed earth or recently twisted earth in your garden.

Sea and reef pools often have a slight smell of the sea or beach.

It is important to know that these are not strong, ubiquitous smells. You should be able to sit in a room in the dark and not guess that there is an aquarium there. So if the pong is strong or overwhelming, something is radically wrong with your aquarium.

What causes bad smells?

There are a few things that cause bad smells in your aquarium, many of which are preventable and all of which are easy to fix.

R.I.P., my fishy friend

Unfortunately, the most common cause of a smelly tank is a dead fish. Once a fish dies, its remains create a foul smell of decay that can quickly fill your room with a distinctive odor.

Even a tiny dead neon tetra or guppy can put a strong punch in the smelly stakes, and a small fish can be incredibly difficult to spot. If you have lots of dense plants and a busy community aquarium, a missing Endler livebearer or molly can easily go unnoticed. As part of your weekly tank maintenance, it's a good idea to regularly check caves, overhangs, rock walls, and other popular hiding spots to make sure nothing has passed without you realizing it.

Of course, the smell could not come from the tank at all. If you hold small eels, snails, or jumping fish like killifish, one of your pets may have escaped, especially if your tank doesn't have a lid or lid. With the exception of some species of eel, which can breathe air, anything else won't live long once it's out of the water and your room will soon start to stink. So check behind your tank, under furniture, and behind sofa cushions (seriously, this actually happened!)

Once you find and remove the deceased fish, the smell will quickly go away.


Unfortunately, overfeeding is a very common problem in the hobby, especially among beginner aquarists. If you feed your fish too much, what your pets don't eat will drift to the bottom of the tank, where it will begin to decompose quickly. As the material breaks down, foul smelling gases are released, with the smell getting stronger as the food debris builds up.

In addition to the unpleasant smell, the decomposition of food causes ammonia to build up in the water, which puts more strain on your biological filter and possibly poisons your fish.

Preventing odors from decomposing fish feed

Preventing bad smells caused by rotting fish food is incredibly easy. The solution to this smelly problem lies in proper management and feeding.

Only offer your fish what they will be eating in a few minutes. Feed your fish twice a day and take one fast day a week if you are not feeding your pets at all. That doesn't harm the fish. In fact, a day without food lets through any material that is still in the fish's digestive tract, preventing health problems like gas and constipation.

Fish waste

All living things produce waste, and your fish are no exception to this rule.

Some fish species also produce more waste than others. For example, goldfish generate a lot of waste, as do large animals like eels and turtles. Provided your aquarium is properly filled and you perform weekly water changes and general tank maintenance, fish waste should not let your tank smell.

Preventing odors from fish waste

There are two golden rules to follow when it comes to avoiding bad smells caused by fish waste.

Don't overfill your aquarium! Typically, you can keep an inch of fish per gallon of water. If you have large body fish like cichlids, or particularly dirty fish like goldfish, it is safer to use an inch of fish for every two gallons of water.

Overstocking often happens inadvertently when newbies to the hobby buy fry without knowing that the cute two inch fancy goldfish they bought will end up being six inches long, with the exception of its flowing fin! So always check the size of the adult fish you buy so that you are not surprised.

If your tank is already overflowing, you'll either need to upgrade to a larger tank or give some of your fish to friends or your local fish shop.

Decaying plant matter

Live plants are a great addition to any aquarium. They help oxygenate and purify the water for your fish and give the habitat a natural appearance.

Unfortunately, like all living things, plants die off and occasionally shed a few leaves, which can cause your aquarium to smell. If dead plant material is left in the aquarium for a long time, it will turn brown or black and become slimy and completely uncomfortable. Algae are also a plant substance and also smell as they die and start to decompose.

Decaying plant material gives off a very unpleasant odor that can make the entire tank stink.

Prevention of odors through the decomposition of plant material

Taking care of your live plants should be part of your regular tank maintenance. As you work on cleaning the substrate and changing the water, use aquascaping scissors to cut off dead leaves and broken stems and remove them from the tank before they can decay.

Make sure your plants have enough light for photosynthesis and add fertilizer or CO2 to your tank if necessary so the plants have all the nutrients they need to stay healthy and grow.

Clogged filters

Regardless of its size, every aquarium should have an efficient filter system.

Your mechanical element of the system is responsible for circulating all of the water around your tank, ideally at a rate of four times an hour or more. The water flows over the biological filter media in the filter system, where ammonia and nitrites are processed by the anaerobic bacteria living in the filter media and the chemical filter removes heavy metals and toxic chemicals from the water.

As the water flows through the filter system, all floating debris and dirt particles are removed by the mechanical filter. Ultimately, all of this debris builds up in the filter media and filter box as thick, smelly sludge, and that can really make your aquarium stink.

Once the filter media becomes clogged with mud and dirt, the water can no longer properly circulate through the system. This means that the beneficial bacteria in the biological filter media cannot do their job properly and the quality of the water in the tank will suffer, which is very bad news for your fish.

Prevent bad smells caused by your aquarium filters

To prevent your filter from stinking, you need to clean it regularly.

This is a simple task that should be included in your aquarium maintenance routine. Once a month, remove the mechanical filter media and wash it in the tank water that you removed from a partial water change. This will remove any accumulated mud and debris. The biological filter media does not need to be washed unless it is also clogged with gunge.

If you have particularly dirty fish or a densely populated tank, you may need to clean the filters more frequently, about every three weeks.

Most filter media is durable, so you will need to replace the filter cartridges or upgrade the media, depending on the filtration system you have. Check the manufacturer's instructions for your filter to find out how often to replace the filter media or cartridges.

Dirty substrate

If the stench from your aquarium smells like rotten eggs, it is sulfur. But how can this happen and where does the sulfur come from?

Fine gravel or sand is compacted over time when it is placed at the bottom of an aquarium filled with water. Small air pockets form inside the substrate, trapping waste. Bacteria feed on the waste, and it is these bacteria that are responsible for that offensive odor as the gas they generate rises through the substrate and water into the air.

Often times, these dead zones go undetected until you move the substrate, possibly when planting new plants or moving tank decorations. As soon as the substrate is disturbed, the air pocket is broken and the gas escapes.

Preventing dead zones from forming in your substrate

Again, preventing the formation of dead zones in the substrate is due to good tank maintenance practices.

During a weekly partial water change, use an aquarium vacuum to move the sand or gravel throughout the tank. As you walk, be sure to cover all areas of the tank bottom, paying special attention to the corners, the bottom of the plants, and the decorations.

Water purifier

Although you will need to use a water conditioner to remove toxic chlorine or chloramine from tap water before adding it to your aquarium, some conditioners can "off" the water in your tank.

However, that eggy aroma caused by sulfur in the conditioner formula will generally go away once the conditioner has spread throughout the tank.

How to refresh your aquarium … and let it smell fresh!

Now that you've identified and removed the cause of the whiffy water in your aquarium, you'll want to freshen up and keep your aquarium that way!

Remove decomposing organic substances

Your first job is to get rid of all of the dead and decaying things in your aquarium, including:

  • dead cattle
  • dead plant matter
  • Fish waste
  • dirty filters

First, take an inventory of all of your fish, shrimp, snails, and other creatures that live in your tank. If someone goes missing, search the tank thoroughly and check all usual hiding places to make sure there are no invisible corpses decaying there. If you find a deceased animal, remove it immediately.

When all of your fish are in place and correct, check your live plants. Cut off dead leaves or broken stems and remove fallen leaves that have fallen behind decorations or are wedged between plant stems at the bottom of the tank.

Thoroughly clean your tank

  1. Use an algae magnet to thoroughly clean the aquarium viewing panels.
  2. Next, use an aquarium vacuum or siphon to thoroughly clean the substrate and remove any fish debris, uneaten food, and plant debris trapped there. Pay close attention to corners where dead zones can form.
  3. Remove the tank decorations and clean them thoroughly. Remove algae and general debris.
  4. Now take out the filter system and clean it thoroughly. Wash the mechanical filter media in tank water to remove sludge that could prevent good flow through the system. Discard and, if necessary, replace used filter cartridges or filter media that is beyond the best.
  5. However, Do not replace all biological filter media at once. If necessary, replace a third at a time so that the biological filter system does not crash and a new nitrogen cycle starts.
  6. Make sure to clean the filter outlet and inlet and check that the impeller is free of dirt and can rotate properly.
  7. Perform a 20% water change.

It still stinks!

If your aquarium was really dirty, it will almost certainly still smell, even after you've done all of the maintenance listed above.

That's because you've most likely kicked up a lot of debris that is now floating in the water. For the next few days, change the water 10% every day to gradually refresh the water.

The water should now appear clearer and the smell should be all but gone. However, for safety reasons, we recommend checking the filter medium again. If the water was filled with floating debris after the first deep wash, there's a good chance the mechanical filter will need to be washed through again.

What about a carbon filter?

Many hobbyists swear by activated carbon as a highly efficient way to remove odors from the tank. Activated carbon filters also ensure that the water remains clear and free from discoloration. Your filtration system may already have carbon filters in the system. If not, you may be able to customize the device to do so.

The main disadvantage to using carbon filters is that they only last about a month before you need to replace them. This is not a problem if you only want to use the filter temporarily to remove the bad smell from your tank after cleaning it. Plus, the carbon also removes most of the fish medicines from the water. So if you want to treat any of your fish, you will need to put it in a quarantine tank for the duration of its treatment.

Final thoughts

So the bottom line is, if your aquarium smells, it is most likely due to poor maintenance and management.

If you keep your aquarium clean, properly maintain the filtration system, and remove all dead organics as soon as you see it, the healthy aroma of your tank should go undetected until you lift the lid and take a deep breath.

Use the tips and information we have provided in this guide to keep your aquarium pristine and enjoy the sweet smell of success instead of the foul smell of decay!

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