A Guide to Get Rid of Aquarium Smell

As soon as you enter the room, a bad, strong smell is all around you. You look at your aquarium and all the peace is gone. Why does your fish tank smell bad, but look like it’s doing well?

If your aquarium starts to smell bad, it can mean that important parts of your tank need attention. Both beginners and advanced aquarists should know what causes bad smells and how to eliminate them. In order for your tank to be healthy and smell-free.

We will walk you through what makes aquariums smell, how to identify the main source and how to get rid of the smell forever.

What Does a Healthy Aquarium Smell Like?

If your aquarium is taken care of, it should have a gentle scent resembling earth, marine, or pond smells. If you smell something faint, it means your water is healthy, the plants are working well, and the bacteria are doing their job.

If your tank starts smelling unpleasant, sharp or foul, that’s a warning sign. It is possible that the smell is signs of decay, pollution or imbalances in your water quality. A tank with a bad smell isn’t pleasing. It also tells you something in your aquarium is out of balance and should be dealt with right away.

Why Does Your Aquarium Smell Bad?

Various reasons can make aquarium water stink, like organic matter or chemical changes. These are the usual things that go wrong:

1. Dead Fish and Plants

If your fish is small or hides frequently, it’s possible to miss its death. Rotting fish quickly become a source of ammonia and other substances that can damage the water.

How to Check:

Count your fish regularly.
Use a net or siphon to check behind decorations and inside caves.
Look for plant matter that is brown, slimy, or disintegrating—these are signs of decay.

2. Fish Feces and Rotting Food

Fish food that is left uneaten often sinks, decays, and causes harmful bacteria that give off bad smells. Too much fish waste is also a problem, particularly in tanks filled with too many fish.

Prevention Tips:

Only feed what your fish can eat in 2-3 minutes.
Use a gravel vacuum to remove leftover debris during water changes.
Observe your fish’s behavior—if food is consistently being ignored, feed less.

3. Dirty Filters and Equipment

If your filter isn’t clean, all parts of your aquarium will struggle. If a filter gathers sludge or debris, it doesn’t work well and can start to smell on its own.

Maintenance Tips:

Rinse filter media in old tank water (not tap water) during water changes.
Replace carbon filters every 2-4 weeks.
Clean tubing and impellers monthly.

4. Compacted Gravel or Sand

Gravel or sand in your tank may keep the waste and food from moving, causing problems. With time, it gets tighter and can support anaerobic bacteria. These bacteria create a strong smell from hydrogen sulfide gas.

What to Do:

Gently stir the substrate with your fingers or a gravel vacuum.
Avoid letting substrate go untouched for long periods, especially in planted tanks.

5. Water Conditioners and Chemicals

Many water conditioners with sulfur or binding agents can briefly cause a smell. It usually doesn’t do any harm, but it might upset people new to the hobby.

Tip:

Use conditioners that are known for minimal odor.
If the smell persists beyond a few hours after use, it might be something else.

How to Identify the Source of the Smell

To pinpoint the cause of a bad smell in your aquarium, use a checklist approach:

Aquarium Smell Source Checklist

Count your fish—any missing?
Look for decaying plant matter or uneaten food.
Check the filter—does it need cleaning?
Stir the substrate—do you notice any trapped debris or smell release?
Consider any recent chemical treatments or new conditioners.

Use Water Test Kits

Testing your water can reveal hidden problems:

1. Ammonia and nitrite spikes often signal decay or overfeeding.

2. High nitrate levels may suggest the tank is overdue for cleaning.

3. pH fluctuations can also affect odor levels indirectly.

Pay Attention to the Type of Smell

Rotten egg = hydrogen sulfide, likely from the substrate.

Sour or pungent = decaying organic matter.

Chemical or bleach-like = possible water treatment or cleaning residue.

Step 1: Deep Clean and Maintenance

Begins by shutting off all electrical gadgets and removing any decorations, plastic plants and accessories. Look for evidence of moss, slime or decomposing things on every object. Wash the decorations with an appropriate brush, rather than washing them with soap that might leave harm behind.

Thoroughly clean the walls of your tank, with a special brush or magnet, since buildup happens most often at the waterline. Following, take care of the substrate by vacuuming or siphoning debris inside. Pay special attention to parts that get filled with debris regularly, for example, corners and under many objects.

Always clean your mechanical filter media in tank water, not tap water. This is to make sure you don’t destroy the helpful bacteria during water changes. When the chemical part of your filter is no longer functioning, replace it with fresh carbon or Zeolite.

Step 2: Water Changes

Add fresh, untreated water to your tank for a 25–50% change to help get rid of dissolved wastes and bring in fresh air. It helps make foul smells less strong.

Change 10% to 20% of the water every few days until the smell has gone away. When your fish are in water, consistently match the new water’s temperature, pH, and hardness to avoid causing them stress.

Use a conditioner for your water that removes ammonia, chlorine and chloramines. If the water was of poor quality, buy products that aid in slime coat maintenance. So, your fish can return to health.

Step 3: Use Activated Carbon

Because activated carbon is so porous, it effectively removes both waste and the compounds that give off smell. Keep the filter in your filter’s media compartment or hang a mesh bag with it in the flow of your water. Change the mixture every 2–4 weeks so it continues to work.

If your tank produces a lot of waste, mix Zeolite (which eliminates ammonia) with Purigen. A material that effectively removes discoloration and bad smells. Using filter pads treated with carbon or media like Chemi-Pure Elite is a good way to treat your home’s water.

Note: Don’t depend completely on these products—you should clean and maintain your car in addition to using them.

Step 4: Address Overcrowding and Overfeeding

If your tank is too full of fish, your filter will not remove all the waste. Keep fish to 1 inch per gallon of water. But you need to ensure you look up the needs of goldfish and cichlids, as they require more attention.

Feed your fish only once a day or every other day, based on the kind of fish you have. Soon after uneaten food enters the water, it decays and releases ammonia along with bacteria that smell bad.

Don’t make the meal larger than what your fish can eat within 2–3 minutes. Take out extra food as soon as you can with a net or siphon. Feeding bottom-living fish with sinking or slow-dissolving food will prevent food accumulation in the substrate.

How to Prevent Aquarium Smells

Once the fresh scent is back, be sure to stick with the care routine to keep your aquarium clean. Here’s what you can do to avoid future odor problems.

1. Establish a Schedule

Let every week have one day dedicated to cleaning tasks. Scrape the glass with an algae remover. Remove brown plant leaves and siphon the substrate to remove waste. Check behind decorations and under rocks as well.

Fill at least 20–30% of your tank with dechlorinated water once every week. As a result, ammonia, nitrite and nitrate are kept low.

Record your water parameters (ammonia, nitrite, nitrate, pH, and temperature) using a notebook or mobile app. Trends can help you catch problems early before odors arise.

2. Monitor Feeding

Never feed your fish more than they can eat in 3 minutes. Anything not consumed will cause more waste and bad odors. If you use flaked or pelleted food, give them just a few pinches and see how they react.

One day a week, avoid putting fish food into the tank to assist in digestion and reduce extra waste. The majority of fish can fast for short periods and find it beneficial.

Both surface feeders and deep-dwelling fish should be fed using the right tools so food does not sink and spoil in places you cannot see. You can use a demand feeder or feed blower for automatic feeding. For manual feeding, opt feeding tongs or feeding sticks.

3. Keep biological balance

Healthy growing plants take up nitrates which promotes stable water and reduces bad smells. Remove dead leaves occasionally. Also cut away extra growth and add root tabs or liquid fertilizer as necessary.
Use a timer that controls light for 8 to 10 hours a day to stop algae growth. Excessive light in the aquarium helps algae grow, which might decay and release foul odors.
Depending on the size and occupants of the tank, you may want to include Amano shrimp, Otocinclus catfish, or nerite snails. They support algae control with little to no waste.

Conclusion

If your aquarium smells bad, it’s usually a clue that something is wrong with your water ecosystem. You might find your tank smelling bad because of rotting food, dead fish, unclean filters or an unhygienic substrate.

But there’s also some good news: You can fix it—and make it so that it doesn’t always happen. By understanding and applying those tips, your tank will not only smell better but also support happier, healthier fish.

Do you have any issues or challenges while setting up your aquarium? Feel free to contact AQQA.