Most clean freshwater aquariums smell like a mild earthy aroma, almost like freshly tilled soil, which is actually good and what all aquarist should strive for.
However, a fish tank scent is generally mild, and you’ll only catch it if near or when working on your fish tank. Any smell strong enough that it’s noticeable in the air inside your aquarium room is an indication something is wrong.
In my experience, the most common unnatural stinks I’ve heard of are sulfur, ammonia and musty whiffs.
That said, today we’ll go through these off-odors occasionally reported in fish tanks; the causes and how to get rid of them.
What Should a Freshwater Fish Tank Smell Like?
Realistically, there is no standard scent for a freshwater aquarium because each tank is unique and everyone’s olfaction works differently.
Nonetheless, this is how I would describe the smell of my clean freshwater fish tank.
My fish tank water, when generally clean and mostly after a water change, has an earthy smell, a lot like a clean pond or lake in the spring.
In which case, the term earthy can also be used to describe the smell of clean tank water, a watered lawn or garden, cut grass after rain or the smell of clean dirt.
But never like sludge, stagnant backwater inlets or dead fish.
Though some tanks will have a general fishy smell from all the bacteria present an aquarium.
Why Does Your Fish Tank Stink?
The smell in your fish tank will not always be pleasant, sometimes it will be the furthest thing there is from an earthy scent, to say the least.
Adverse odors in fish tanks are usually linked to biowastes, mostly from decomposing fish, excess food waste or rotting aquatic plants.
So, let’s look at these biowastes concisely.
#1 — Dead Fish
This is by far the most common cause of a stinky tank, especially if the odor is strong enough you can smell it in the air. And the stench is worse when it’s a snail that died in the tank.
Moreover, if a fish dies when you’re away, maybe on vacation, it will fill your room with the awful odor.
More appalling is that for you to notice the stench, your fish must have died several days before. At least long enough for it to start decomposing.
However, sometimes you’ll be home but not notice a dead fish because it’s quite hard to see inside a tank that’s closely planted or one full of decoration. So, you’ll want to keep your tankless cluttered.
An aquaria-wise chop is to do a fish roll call every once a while and keep an eye on sickly fish. Also, look out for lifeless bodies that’ll tend to float gradually.
Since fish muscles and spine bones are heavier, dead fish float belly up.
Other times you’ll miss a fish and a body, in which case you’ll need to look outside the tank because some fish jump out to their demise. This especially if your tank has no lid.
#2 — Excess Food Waste
By and large, though weary of starving your fish, try not to overfeed them because they can only eat so much in one feeding and waste the rest of the food.
You should generally feed your fish food they can finish in a few minutes and remove any leftovers after a short while.
Leaving uneaten food in the tank will result in foul smell once the scraps fall to the bottom of your tank. This leftovers ordinarily get stuck in the substrate and promote bacteria overgrowth which then releases the stinky gases.
As well, when the scraps decay, they let out a foul smell like any other organic matter.
A good cleaning and water change routine, a powerful filter and stocking scavengers like shrimp, cory catfish, or plecos (bristlenose) are all ideal solutions to any grime stuck in the substrate.
#3 — Fish Waste
Fish waste generally smells like Ammonia, but the stench is mostly mild, and only the smell from excess waste is discernably strong.
So, if you max out your fish tank, chances are you aquarium will stink and you’ll have water quality issues. Your aquarium will occasionally have a strange which will disappear when you do a 20-30 percent water change.
Also, if your filter might not be working properly from one of many reasons which include dirty clogged equipment.
If you have this foul ammonia smell, recalculate the dimensions of your tank to find out whether you are overstocked. And if that’s the case consider moving some of your fish to another tank.
Then make sure your filter is adequate for your tank size, also clean the filter and unclog the inlets where need be. Mostly, your filters will block when you have a sand substrate.
Sometimes, your tap water will have a lot of nitrates which though won’t smell, in excess amount will gradually cause liver and eye problems to your fish.
Plus excess nitrates couple with too much light will cause an algae overgrowth.
One other efficient hack is to add live plants in your tank to naturally clean the aquarium.
#4 — Decomposing Plants
Though adding plants in an aquarium should generally clean the tank, a rotting plant can give off an awful smell if left unattended, though this is less common than other causes.
Plus dead plants are easy to spot as they often turn a slimy brown or black and foul up the water quality.
Why Your Fish Water Smell Like Rotten Eggs (Sulfur)
If your fish tank smells like rotten eggs, which is basically the smell of sulfur, it means you have a weighty bacteria buildup problem mostly coupled with organic waste stuck in the substrate.
Basically, the fine gravel or sand used to base freshwater aquariums, compact over time and form zones with insufficient oxygen supply.
As waste accumulates in these spaces, it overstimulates bacteria growth which gives off a smelly gas.
The bacteria, in this case, are anaerobic which have low dissolved oxygen and produce Hydrogen Sulfide with the signature rotten eggs stench.
In fact, the bacteria is present in most low oxygen aquatic environments like swamps and that’s the reason some aquarists refer to the smell as swampy or musty.
This problem escalates when the air pump or filter in your tank is dirty or faulty, which also build-up nitrates and phosphates levels and in turn feed the bacteria.
To remedy the situation, you’ll need to vacuum your substrate frequently, and also make sure your filters and air pump are clean and working properly.
In case the smell is too much, empty your tank and do a thorough cleaning of all surfaces and equipment.
Why Your Fish Tank Smells Like Mold
You may have noticed a moldy smell in your fish tank but the cause is most likely not mold but algae or fungus. Though not to say mold can’t grow in an aquarium, it’s just highly unlikely.
On the other hand, blue-green algae (cyanobacteria) are common and though not always they tend to have a moldy smell.
Biologically speaking, blue-green algae are not algae but bacteria. And there are hundreds, if not thousands of species which may manifest different traits.
If the smell is indeed coming from blue-green algae, the odor will be accompanied by a visible film on aquarium glass, decorations, rocks, driftwood and even on fish.
This film will appear green and slimy or may have a brown appearance in which case that would be brown algae (diatoms).
That said, sometimes algae will remain odorless if you don’t touch it, but once you start taking it out the whole room will smell.
To get rid of algae, physically remove the substance c with a thorough cleaning but you must resolve any water chemistry problems to keep mold from returning.
Why Your Fish Tank is Cloudy and Smelly
There are a couple of reasons why a fish tank can get cloudy. From uncleaned gravel to dissolved nutrient, but mostly it’s because of bacteria bloom.
As nitrifying bacteria establish in a new tank, they spread through the water in a hazy cloud which should clear up naturally.
However, the bacteria do not cause the smell per se. The unfiltered dirt, including ammonia, that accumulates before the beneficial bacteria fully develop creates the odor.
To get rid of the smell, do frequent water 50 percent water changes and clean the fish tank once every week. In case your schedule is full, you can clean the tank every two weeks.
Also, cut back on the feeding and especially don’t overfeed your fish.
Nonetheless, a dirty substrate will also cause cloudiness and foul smell in the water even in older tanks with established bacteria.
How to Treat Stinky Aquarium Water
Each smell in an aquarium might be unique but you’ll follow the same steps towards a solution.
First, you have to find the source of the smell and remove it then use proper cleaning fixes to remove the lingering odor.
Take a roll call of your fish to make sure everyone is alive. In case you have any that is sickly or lethargic, find out what’s ailing it and if serious, consider placing it in a quarantine tank.
If there are no dead fish in the tank, move on to your live plants. First, consider obvious dead leaves and take them out. You’ll also want to prune leaves showing any signs of weakness or those that are overgrown and risk cluttering the tank.
In case there are blue-green algae growing in the tank, physically remove them from plants and decorations. Sometimes, you may have to lift the decorations dust and clean them outside the tank.
Also test your water for nitrate, phosphorous and silicate and pH. Algae growth is equally encouraged by too much light, so don’t expose your tank to too much sunlight. In the case where you use aquarium lights, only leave them on for a maximum of 12hours each day.
After you’ve dealt with the source of the smell, its time to establish an odorless environment in the tank.
Do regular water changes over the next couple of day.
Preferably, replace 10 to 20 percent of the old water with fresh odorless water every day for about a week. In case the previous water test showed high levels of dissolved minerals, you may want to switch from tap to RO water.
However, RO water is too pure for use in aquariums, it lacks most minerals that are beneficial to aquatic environments. Therefore, remineralize the water prior to using it in your tank.
Then over that one week, do regular water tests and compare with the previous ones and continually try to improve the quality.
One last thing, make sure your tank is not overstocked because no matter how much cleaning and water changes you do, the aquarium will always get toxic faster than you can clean it.
Cleaning and Maintenance
There are a couple of things you’ll want to clean in a stinky fish tank starting with the substrate.
Proper vacuuming will first remove any uneaten food and organic plant waste stuck in the gravel. Plus the movement created by the siphon kit will decompact the substrate and break any oxygen-free pocket that would otherwise encourage anaerobic bacteria growth.
While still cleaning the bottom of the tank, take a minute to inspect your airstones and under gravel tubing. Make sure the system is oxygenating your fish tank properly, then shift your focus to your aquarium filter.
What is more important even before you find out whether your filter is clean, is to make sure its sufficient for your fish tank. Generally, sponge filters are affordable but not powerful enough for bigger tanks.
If you suspect your filter is underperforming, consider getting a more effective kit or buy a second filter to supplement the one you have.
Keeping that in mind, if you are confident your filter is performing optimally, then all you need to do is make sure it’s not dirty. Clear any grime in the inlets and outlet that might be clogging the filter.
However, clean your filter about a week after you are done with the water change. This will give your fish enough time to adjust to the new conditions before another major change in the fish tank.
The other thing to remember is the cleaning method you use will depend on the type of filter you have. Consult the owner’s manual for any specific direction for your filter type.
Use Carbon Media in Your Filter
With a clean operational filter, you can use a carbon filter to reduce the stench, but you have to change it regularly.
Carbon filters work by adsorption to trap pollutants in the carbon media. And in the same way, they’ll pull odors from your aquarium and stop them from filling your space.
Nonetheless, make sure you buy quality activated carbon for use in your tank. Cheap carbon often contains more phosphates and ash hence become deactivated quickly.
It’s also important to have a carbon material with a large surface area because this provides more bonding site to catch odors and other pollutants.
Enjoy you odorless aquarium