A well-maintained and clean aquarium can enhance any interior decor. Unfortunately, in some cases, aquarists may encounter the problem of unpleasant smells signaling various issues.
There are numerous reasons why an unpleasant odor can develop in an aquarium, but they all ultimately come down to high levels of decomposing organic matter, subpar equipment, mold, and the presence of algae (cyanobacteria).
In this article, I will provide a detailed description of what can lead to all the aforementioned causes and what we can do to fix them.
What Causes the Unpleasant Smell?
In normal conditions, a well-maintained aquarium should not have a strong or unpleasant smell. In fact, a properly cared-for aquarium should have little to no noticeable smell at all.
However, this is not always the case, especially for novice aquarium enthusiasts.
The unpleasant smell can be caused by various factors such as:
- Overcrowding (Excessive Animal Waste)
- Overfeeding
- Dead animal
- Decomposing plants
- Inadequate aeration
- Irregular or Inadequate Cleaning
- Filter issues
- Driftwood Problems and Poor Quality Décor
- Bacterial Blooms (Cloudy Water)
- Inadequate Lighting
- Lid smell
- New Tank Smells
- Low-Quality Food
- Tap water problems
- Infrequent Water Changes
Therefore, first and foremost, it’s important to determine the problem.
1. Overcrowding (Excessive Animal Waste)
This is, probably, the main cause of foul aquarium odors. Overcrowding occurs when the volume of water does not match the number of animals inside.
| Fish, crayfish, crabs, axolotls, frogs, and snails produce lots of waste that foul the substrate and water. |
It is important to remember that each animal requires adequate space for a healthy life. If the tank balance is off, the water often becomes cloudy and greenish, with a swampy stench.
Very experienced aquarists can often estimate when an aquarium is becoming overcrowded just by looking at it. However, for most cases, especially beginners, it’s better to use aquarium calculators such as.
This will provide you with an initial understanding of the situation and help you make the right decisions.
Solutions: Control your livestock and/or increase water volume (buying a larger tank).
Temporary solutions: increasing aeration, increased water changes, and regular aquarium cleaning.
2. Overfeeding
In my opinion, this is the second most common cause of foul aquarium smells. Beginner aquarists do not understand that it is better to underfeed than overfeed.
Overfeeding can easily harm the tank in many ways including:
- Bad water quality (unstable water parameters)
- Diseases and Infections
- Parasites infestations
- Cloudy water
- Algae growth
- Reduced oxygen levels
- Smelly water
Uneaten food that sinks at the bottom of the tank and decays promotes microbial growth that disrupts the water’s biological balance, increasing oxidability. The smell from overfeeding is rotten and pungent.
With proper feeding amounts and cleanup of any excess, foul aquarium odors can be easily avoided.
Solutions: Reducing portion sizes and/or less frequent feeding, removing uneaten food, and siphon cleaning the substrate after accidental overfeeds.
Temporary solutions: increased water changes and regular aquarium cleaning to remove uneaten food.
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3. Dead Animals
In decorated or heavily planted tanks, it can be hard to notice dead animals right away. In some cases, it becomes practically impossible to do it if, for example, it happened with a snail that buried itself in the substrate.
Note: Examples of burrowing snails – Assassin snails, Mystery snails, Rabbit snails, Black Devil snails, Malaysian Trumpet snails, White Wizard snails, Asolene spixi, and Chopstick snails.
Dead animal not only smells foul but also risks serious infection spreading. Dead animals release ammonia as they decompose, which is toxic. Their decaying body is a breeding ground for dangerous bacteria and fungi that enter the water.
The smell of a dead animal is rotten. It usually starts spreading if a dead animal has been in the aquarium for 12 hours or more.
Solutions: Remove the dead animal immediately and do a 30-40 % water change.
| Important: Snails can be harmed by introducing medications containing copper, treatments for combating planaria, etc. When there are a large number of snails in the aquarium, such medications should be used with great caution. |
4. Decomposing plants
Much like with dead animals, decomposing plants can also lead to unpleasant odor buildup in an aquarium.
Dying and decaying plant matter releases a variety of organic compounds into the water as it breaks down. The decomposition process consumes oxygen, allowing anaerobic pockets to form and release a foul smell. Additionally, decaying plants are susceptible to fungal growth and rotting from species.
Solutions: Regularly pruning and removing any dead or dying plant matter before it begins decomposing is key. Providing plants with optimal light, nutrients, and care reduces die-off.
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5. Inadequate Aeration
Aquarium water should never be stagnant.
Proper aquarium aeration supports the gill respiration of fish and invertebrates while preventing the growth of anaerobic bacteria that can produce unpleasant odors. Additionally, some animals can be filter-feeders (Bamboo shrimp, Vampire shrimp, Freshwater clams, etc.) and require suspended particles in the water column.
If you have a low to moderate stocking tank and its height does not exceed 15 – 20 inches (40 – 50) cm, the filter-generated current is often sufficient to effectively mix the water and oxygenate it.
Note: Of course, it also depends on the animals you keep.
However, the situation completely changes in large, tall, narrow tanks where additional aeration from an air pump becomes absolutely necessary to prevent oxygen deficiency. Without it, there is a very high chance that you notice a foul or even putrid smell on the water’s surface.
Solution: Increase aeration (adjust airflow or install a powerful, high-quality filter and air pump).
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6. Irregular or Inadequate Cleaning
As I have already said before, inadequate cleaning can lead to a buildup of waste and uneaten food in the tank, which can degrade water quality. This can result in high levels of ammonia and nitrites, which are toxic to aquatic inhabitants.
Additionally, irregular cleaning can lead to the accumulation of debris and detritus in the substrate, creating pockets of anaerobic bacteria that produce toxic gases. These gases can harm fish and other aquatic life.
| Substrate problems
Essentially, it’s related to insufficient aquarium maintenance but the problem is often overlooked. 1. Let’s say you have a deep substrate with few or no rooted plants and do not have animals that disturb it periodically (such as snails). Over time, this substrate can become compacted, leading to the formation of anaerobic zones where hydrogen sulfide gas can accumulate. So, when you suddenly remember that it’s time to clean this substrate, a foul odor can quickly arise. If your substrate hasn’t been siphoned for a long time, you can check for the presence of hydrogen sulfide by simply poking it with your finger. If bubbles start to rise, you’ll also immediately notice the smell of rotten eggs. 2. A particular danger in this regard is posed by a coarse substrate (such as gravel or rocks). In such a substrate, leftovers, debris, and other organic matter can easily sink into and decompose, often leading to increased levels of ammonia, nitrates, and a foul odor. |
Solutions: clean the walls, vacuum the substrate, and do a water change.
Additional solution: introduce clean-up crew
7. Filter Issues
If your aquarium smells, it will be a good idea to check your filter. Here are some reasons for that:
- Accumulation of debris. Filters can get clogged with uneaten food, fish waste, and other organic debris. As this material decomposes, it releases unpleasant odors.
- Buildup of nitrates. Insufficient filtration allows ammonia and nitrites to accumulate from the nitrogen cycle. High nitrogen levels encourage algae and bacteria growth, adding to the stench.
- Lack of oxygenation. Clogged filters or weak pump flow lead to low oxygen circulation (I have mentioned earlier). This allows anaerobic “dead zones” to form, releasing hydrogen sulfide gas which smells like rotten eggs.
- Recirculating odors. Dirty filter media continually recycles stinky water through the tank instead of removing odors during cleaning.
Solutions: Regular filter maintenance like rinsing media in old tank water, replacing cartridges per manufacturer instructions, and cleaning intake tubes prevent many issues. Additionally, using properly sized, high-quality filters suitable for the tank size and stocking level is also key to maintaining good water quality and avoiding unpleasant aquarium odors.
| In conversations with other aquarists, I’ve heard that some even turn off their filtration during feeding.
They argue that by doing so, fewer leftover food particles find their way into the filter, preventing them from decomposing and causing unpleasant odors. Well, while they do have a point, it’s not the whole story. The issue is that these leftover food bits, which would have otherwise been filtered, now accumulate on or beneath the substrate and can still lead to undesirable smells. Tip: Do not feed your animals near filtration and use water dishes if possible. |
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8. Driftwood Problems and Poor Quality Decor
These are other reasons for unpleasant odors.
Poor quality decor can release toxic substances into the water. This often leads to an unpleasant chemical odor.
In aquariums, new driftwood will leach a lot of tannins when first submerged, causing a stronger smell (not too unpleasant though). This odor should fade over several weeks as the wood becomes waterlogged.
However, the smell can become extremely unpleasant, and in such situations, it’s essential to identify the underlying causes.
- Anaerobic pockets. Dense wood can have air pockets that go anaerobic, trapping decomposing debris and releasing hydrogen sulfide gas, causing a rotten egg smell.
- Softening and decay. Over time (it can be months and years) but eventually, driftwood will decay (very porous wood will break down faster), fouling water, creating more anaerobic pockets and fungi.
Solutions:
- Use only aquarium-safe products.
- Pay attention to the appearance of the products, the quality of the paint, and the presence of adhesives.
- Denser woods like mopani release fewer tannins and resist decay longer. Clean and boil it (curing process – read more about it here).
- If driftwood decays, you need to take it out and throw it
9. Bacterial Blooms (Cloudy Water)
Bacterial blooms can definitely cause unpleasant smell in our tanks as well. Blooms involve the explosive reproduction of bacteria. Their massive increase disrupts tank balance.
- Organic waste decomposition. The bacteria rapidly break down any excess fish food, plant matter, or other organics, releasing sulfur compounds and other smelly byproducts.
- Low oxygen conditions. High bacterial activity can deplete oxygen levels, leading to anaerobic pockets that produce hydrogen sulfide gas, which smells like rotten eggs.
| New tank cycling. During the nitrogen cycle, bacterial blooms are common as populations explode, causing stench until the system matures. |
Solution: Reduce feeding, do water change, vacuum the substrate, and add a piece of filter wool to your filter. It works by trapping particles of debris that are in the aquarium column.
Temporary solution: Flocculants can be used to clear cloudy water. It makes the debris or bacteria causing the cloudiness to clump together thereby making it easier for the filter to take them out of the water.
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10. Inadequate Lighting
Inadequate lighting can also contribute to foul aquarium odors.
With too much light, nuisance algae like cyanobacteria thrive. They create a musky odor as they rapidly proliferate. The duration of light exposure is also important.
Solution: Control the lighting, add plants to compete with algae and cyanobacteria.
11. Lid Smell (Mold)
So, you have done all the cleaning, added aeration, reduced feeding, checked everything, and yet the unpleasant smell (like mold) lingers. Sometimes the problem is not in the water.
Have you cleaned the lid and the cover glass?
Typically, this occurs in older, long-established aquariums that have lids. Under the lid, beneath the latches, and along the walls, where water levels may have dropped, mold spores can develop. Such ideal conditions (high humidity, warmth, and dirt) provide a boost for its spread in various areas, often in hard-to-reach spots.
Bits of food might also stick between the glass over time, contributing to the odor.
Many people overlook this, but both the lid and the cover glass can absorb smell like sponges.
Solution: Clean the lid. You need to address all these factors as well. Don’t forget about these little details.
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12. New Tank Smells
Brand new aquariums, especially inexpensive acrylic or plastic kits, may have an unpleasant plastic or chemical (because of poor quality silicon) odor at first.
This is due to residue left on the tank walls and accessories from manufacturing. Components like filters, tubing, gravel, and foam padding can also emit smells as they leach chemicals into the water initially.
The plastic polymers and chemical compounds used to make these parts can vary between brands and batches. So while one new tank setup may have no odor, another could have a very strong smell.
Solution: Thoroughly clean the tank and rinse all equipment before setup. If you have already filled the tank, do several water changes over the first few days and wipe down all surfaces. It may take from a few days to a few weeks but the smell will go away.
13. Low-Quality Food
Fortunately, this does not happen very often, and even lower-quality food typically doesn’t produce strong odors.
Nevertheless, it’s worth keeping in mind, especially if you have decided to try a new type of food for your pets and you start noticing an odor.
| One of the signs of low-quality fish food is its quick “disintegration” in water, where pellets or flakes rapidly break down into numerous tiny particles that fish simply can’t consume in time.
As a result, these tiny particles settle at the bottom or accumulate in the substrate, filter media, and decay there. |
Solution: Stop using new food for the next 3-5 days. Do water change and vacuum the substrate. If the problem was in this product, the smell would disappear.
14. Tap water problem
Let’s not forget about tap water. Sometimes, municipal authorities undertake upgrades, replacements, or routine maintenance on water supply systems.
As a result, the tap water you are accustomed to may exhibit different characteristics, including elevated levels of organic matter, metals, chlorine, and chloramines.
- Chlorine and Chloramines: Many municipal water supplies use chlorine or chloramines to disinfect tap water. When added directly to the aquarium without dechlorination, these chemicals may disrupt the beneficial bacteria that break down waste. As a result, this disruption can lead to foul odors, often resembling a strong bleach-like smell.
- Ammonia and Nitrates: High levels of nitrogen in tap water can contribute to smell in the aquarium.
- Very Hard Water: Tap water with too high mineral content can lead to the buildup of scale and deposits on aquarium surfaces (glass, heater, decorations, etc.). Over time, these deposits can harbor bacteria and contribute to odor problems.
Solution: Regularly check your tap water parameters, let it age for a few days, use a water conditioner, periodically clean and remove any mineral deposits, and do water changes.
15. Infrequent Water Changes
Unless you have a specific build setup, you need to do regular water changes.
Otherwise, insufficient water changes will lead to odor buildup primarily due to the accumulation of dissolved organic waste and the breakdown of uneaten food and fish waste.
Solution: The solution is scheduled, adequate water changes like 10% for shrimp tanks and 30-40% for fish tanks weekly.
Related articles:
- No Water Change Tank?! Top Offs vs Water Change
- How to Do and How Often to Do Water Change in Shrimp Aquarium
In Conclusion
There are many reasons why aquarium smells. Therefore, you will have to investigate step by step, considering each of the reasons listed above.
Personally, I would start with overstocking, overfeeding, and aquarium maintenance. Since practically all the other causes are actually derived from these to one degree or another.
The aquarium hobby is not for lazy people. A common misconception out there is that aquariums are a ‘set it and forget it’ kind of deal, but that couldn’t be further from reality. The choice is yours.