What’s that bad smell?
Is it coming from your aquarium?
If it is, it could be a problem…
Now don’t get me wrong, a healthy fish tank will have a slight smell.
No, it isn’t a fishy smell. Rather, a pleasant smell. Here are how fellow fish keepers describe the scent of their tanks…
- Freshwater tanks – A mild earthy aroma, like freshly tilled soil
- Saltwater tanks – A slight ocean scent, like the beach
Hardly what you would call bad smells, right? I mean, you can buy candles with those scents!
The thing I must stress is that these scents are not strong. You should never close your eyes and guess that there is a fish tank in the area.
If you can smell your fish tank from across the room, and it stinks, there could be something seriously wrong with your aquarium.
Today, I am going to take you through all the common causes of these smells as well as how to remove any overpowering odors from your aquarium.
So, your fish tank smells funny, huh? Well, chances are that one of the following 7 causes is the culprit…
Causes of bad smells
1. Something died
I hate to break it to you, but that foul smell coming from your fish tank could be the remains of a much-loved fish. A rotting fish stinks and can fill your room with a horrible odor.
If you have a tank full of decorations or plants, a dead fish can be difficult to spot. And if you have a crowded community tank, a missing fish can easily go unnoticed.
Heck, I once had a common pleco that was shy and only came out at night. He spent his time hidden away in the corner of the tank. Days went by without me seeing him.
Well, he gave up the ghost, and I was none the wiser until a week later, when I noticed a musty smell coming from the tank. Upon investigation, I discovered his decomposing body deep inside his favorite hiding spot.
So now, it’s time for some fish roll call. Any missing fish could very well be the source of the strange smell. Be sure to check behind every rock, decoration and plant. You would be amazed at the tight spaces that fish can squeeze themselves into.
Your dead fish might not even be in the tank at all! This is common with jumping fish, like killifish, who can easily leap out of aquariums that don’t have a lid. Needless to say, a fish out of water won’t last long and will soon begin to stink.
If your fish can jump, check behind your aquarium stand and any nearby furniture to find the body.
Fortunately, a dead fish is an easy smell to control. The smell should soon disappear when the fish is removed.
How to prevent bad smells caused by dead fish
Unfortunately, this is the most difficult type of smell to control – no fish lives forever. It’s sad but true.
The best thing you can do is give your fish a long and healthy life by getting the essentials right. Good water quality, not overstocking, proper diet and regular maintenance go a long way to seeing your fish live a full life.
I know you don’t want your fish to starve, but overfeeding is perhaps one of the worst things you can do to your fish. It can also be responsible for the horrible smell coming from your aquarium.
You see… Your fish are only going to eat so much food in one feeding session. Anything that is uneaten falls to the bottom of your aquarium and begins to rot.
As the food breaks down, it releases gasses, causing a foul odor. The smell becomes stronger as this uneaten food builds up.
A good tank maintenance routine, such as filter cleaning and using a gravel vac, should remove most traces of decaying fish food and the smell along with it.
How to prevent bad smells caused by too much fish food
It’s actually very simple – only give your fish the amount of food they eat in a single session. This way there will be no fish food left to give off bad smells.
Figuring out just how much requires trial and error on your part – with so many different species of fish and different types of fish food, the exact amount can vary dramatically.
A rule of thumb is to feed your fish only as much as they can eat in 5 minutes.
3. Fish poop
That bad smell could also be caused by the food that your fish do eat.
If your tank is stocked correctly, you will perform your weekly maintenance routine long before the poop begins to stink up your tank.
As you are likely aware:
More fish = More poop
The built-up poop quickly breaks down. As it does, it gives off a foul odor.
How to prevent bad smells caused by fish poop
This one is an easy fix! Don’t overstock your fish tank and only keep fish that are appropriate for your sized tank.
Overstocking is a problem commonly experienced by beginners who do not know better. Before you buy fish for your aquarium, use a stock calculator like this one or ask an expert for their opinion.
If it’s too late for that and your tank is currently overstocked, you have two options:
- Buy a larger tank
- Remove some of your fish
4. Rotting plants
While it’s less common than the other causes on this list, a rotting plant can give off one heck of a stench if left unattended.
Dead plants are easy to spot. They no longer look like themselves and often turn a slimy brown or black color.
You should remove dead plants anyway because they also foul up the water quality.
If you have a plant that only has a few dead leaves, some light pruning is all it takes to remove the rotting bits.
How to prevent bad smells caused by dead plants
Keep them alive! Living plants don’t give off bad smells.
I know, I know… This is easier said than done. But by ensuring the plants have adequate light and nutrients, you give them the best chance to thrive in your aquarium.
Otherwise, why not invest in a clean-up crew for your tank? Malaysian trumpet snails happily devour any dead or decaying plants without any effort on your part.
5. Dirty filter
You know all the things mentioned so far? Well, every one of them can be sucked into your filter system.
In fact, it’s not uncommon to find more rotting sludge in the filter than in the aquarium itself. After all, the job of a mechanical filter is to gather any gunk that is floating through your aquarium and collect it in one place.
And because of this, filters can smell bad. Really bad!
How to prevent bad smells caused by your aquarium filter
I’ll be straight up – cleaning your filter should already be a part of your regular maintenance routine…
When the time comes to do a water change, rinse the filter media in the water you removed so that you don’t kill any good bacteria that are living on it. Doing so should remove most of the stinky sludge that has been stinking up the joint.
6. Your substrate
If the odor coming from your fish tank smells like rotten eggs, then your substrate might be at fault.
This concept is quite complex, so here is the beginner-friendly explanation:
Sand or very fine gravel that has compacted over time can produce zones where there is no oxygen. As waste gets trapped in these pockets, bacteria convert it into gases. Smelly gasses!
This gas then rises up through the aquarium and straight into your nostrils!
How to prevent your substrate from releasing bad smells
Once again, maintenance is the name of the game.
A deep gravel vacuum or regularly stirring sand may prevent these smelly pockets of gas from building up in your aquarium.
Before you select a substrate for your aquarium, read up on how to maintain it!
7. Water conditioner
The water conditioner you add during your water change could also be responsible for making your tank smell funny.
Let’s take my favorite water conditioner Seachem Prime. There is no mistaking the eggy smell it gives off when you open the cap on the bottle.
However, once you add the conditioner to your water, the smell soon fades to the point of being unnoticeable.
How to prevent bad smells caused by water conditioner
Avoid water conditioners with smelly ingredients like sulfur, which is often the cause of that rotten egg smell.
How to remove bad smells
Important: Before you read this step, make sure you have identified the cause of your bad smell from the list above. If you do not remove the cause, the following methods will not fix the lingering odor.
Step 1: Maintenance
First, you want to clean your tank. Your aim is to remove every piece of rotting organic matter in your aquarium.
When trying to remove bad smells, I highly recommend being more thorough than during a regular maintenance routine. So, set aside extra time to do the following:
- Wipe down the glass
- Clean your substrate
- Prune any dead leaves off plants
- Clean any rocks or decorations
- Clean your filter inflow and outlet
- Perform a partial water change
- Rinse your filter to remove trapped gunk
… And any other maintenance steps unique to your tank. If it looks like rotting gunk, and it doesn’t belong in your tank, get it out!
If you have not performed maintenance in some time, it’s very likely that your tank will smell worse after you clean it.
The reason for this is that you are dislodging all the rotting gunk, allowing it to float freely in the water. This should only be temporary. By monitoring your water quality with a test kit and sticking to a strict maintenance schedule, you should knock out the bad smells in no time.
But to speed the process up…
Step 2: Daily water changes
Over the next few days, perform a 10-15% water change. This will add fresh, odorless water to your aquarium while removing some of the foul smelling water.
Step 3: Add a carbon filter (optional)
While you are waiting for the bad smell to clear up, you can add a special ingredient to your filter to stop the bad smells in their tracks…
When it comes to absorbing bad smells, activated carbon works wonders. Also known as a carbon filter, this magical substance pulls odors from your water and stops them from reaching your nose – it will also make your water clearer, removing discoloration.
The only downside is that a carbon filter needs to be regularly replaced, at least once every month. Once the carbon has absorbed all it can hold, the bad smell will return.
Check out our detailed guide on activated carbon filters for more information.
If you have made it this far, you might have noticed a pattern to preventing bad smells in your aquarium.
Did you notice it?
Performing proper and regular maintenance!
You see, almost all bad smells come from rotting organic matter. And with proper maintenance, these should be removed long before they build up and stink up your room.
So, do yourself a favor and set up a maintenance routine. Your nose will thank you for it!
Got a tip for getting rid of bad smells? Let me know in the comments below!
I’ve been keeping fish for over 30 years and currently have 4 different aquariums – it’s an addiction. I’m here to teach you everything there is to know about fishkeeping.
I also use this site as an excuse to spend lots of money on testing and reviewing different aquarium products! You can find my reviews here.