Fish Tank Smells

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Most fish tanks are relatively smell-free. However, sometimes aquarium owners notice unusually stinky tank water, and that shouldn’t happen. Basically, if you can smell your fish tank when the lid is closed, and you’re in another room, there’s a problem that requires your immediate attention.

In this guide, we take a closer look (or sniff!) at fish tank odors to find out what’s causing the pong and how you can fix and prevent those nasty whiffs!

Does a healthy fish tank smell?

Yes! A healthy fish tank does have a slight aroma. The smell isn’t fishy or foul; most hobbyists actually describe it as rather pleasant.

Freshwater tanks have a slight, earthy odor, rather like freshly plowed earth or recently turned soil in your garden.

Marine and reef tanks often have a slight scent of the ocean or the beach.

The important thing to know is that these are not powerful, all-pervasive smells. You should be able to sit in a room in the dark and not be able to guess that there’s an aquarium in there. So, if the pong is strong or overpowering, there’s something radically wrong with your fish tank.

What causes bad smells?

There are quite a few things that cause bad smells in your fish tank, many of which are preventable, and all are easily fixable.

R.I.P., my fishy friend

Unfortunately, the most common cause of a stinky tank is a dead fish. Once a fish dies, its remains will generate a foul stench of decay that can rapidly fill your room with an unmistakable smell.

Even a tiny dead neon tetra or a guppy can pack a powerful punch in the stinky stakes, and a small fish can be incredibly difficult to spot. If you have lots of dense planting and a busy community aquarium, one missing Endler’s livebearer or Molly can easily go unnoticed. As part of your weekly tank maintenance regime, it’s a good idea to routinely check caves, overhangs, rockwork, and other popular hiding places to make sure that nothing has passed away without you realizing.

Of course, it could be that the smell isn’t coming from the tank at all. If you keep small eels, snails, or jumping fish such as killifish, it could be that one of your pets has escaped, especially if your tank doesn’t have a cover slide or a lid. With the exception of some eel species that can breathe air, anything else won’t live for long once it’s out of the water, and your room will soon begin to reek. So, check behind your tank, under furniture, and behind sofa cushions (seriously, that’s actually happened!)

Once you’ve located the deceased fish and removed it, the smell will quickly disappear.


woman feeding fish

Overfeeding is sadly a very common problem in the hobby, especially with beginner aquarists. If you give your fish too much food, what your pets don’t eat will drift down to the bottom of the tank, where it will quickly begin to decompose. As the material breaks down, foul smelling gasses are released, the smell becoming stronger and stronger as the leftover food accumulates.

As well as the unpleasant odor, decomposing food causes ammonia to build up in the water, placing an increased burden on your biological filter and potentially poisoning your fish.

Preventing smells caused by decomposing fish food

Preventing bad smells that are caused by rotting fish food is incredibly simple. The solution to this smelly problem is all down to proper management and feeding practices.

Only offer your fish what they will eat in a couple of minutes. Feed your fish twice a day, and incorporate one “fasting” day per week when you don’t feed your pets at all. That won’t harm the fish. In fact, a day without food allows any material that’s still in the fish’s digestive tract to pass through, preventing health problems such as bloat and constipation.

Fish waste

All living creatures produce waste, and your fish are no exception to that rule.

Some fish species produce more waste than others, too. For example, goldfish generate an awful lot of waste, as do large animals, such as eels and turtles. Provided that your aquarium is stocked correctly and you carry out weekly water changes and general tank maintenance, fish waste shouldn’t make your tank smell.

Preventing smells caused by fish waste

There are two golden rules to observe when it comes to preventing foul odors that are caused by fish waste.

First, don’t overstock your aquarium! As a general rule, you can keep one inch of fish per gallon of water. If you have large-bodied fish, such as cichlids, or particularly dirty fish like goldfish, it’s safer to go with one inch of fish per two gallons of water.

Overstocking often happens accidentally when newbies to the hobby buy juvenile fish, not realizing that the cutesy two-inch fancy goldfish they bought will ultimately grow to measure six inches long, excluding his flowing finnage! So, always check the adult size of the fish you buy so that you don’t get caught by surprise.

If your tank is already overstocked, you’ll either need to upsize to a larger tank or pass on some of your fish to friends or your local fish store.

Decaying plant matter

woman cleaning fish tank

Living plants are a great addition to any fish tank, helping to oxygenate and purify the water for your fish, as well as providing a natural look and feel to the habitat.

Unfortunately, like all living things, plants die off and shed a few leaves occasionally, and that can make your fish tank smell. If left in the fish tank for long, dead plant matter turns brown or black and becomes slimy and thoroughly unpleasant. Algae is also a plant matter, and also smells if it dies off and begins to decompose.

Decaying plant matter gives off a very unpleasant odor, which can make the whole tank stink.

Preventing smells caused by decomposing plant matter

Looking after your living plants should be a part of your regular tank maintenance regimen. As you work on cleaning the substrate and changing water, snip off dead leaves and broken stems with a pair of aquascaping scissors and remove them from the tank before they have a chance to decay.

Make sure that your plants have enough light to photosynthesize, and add fertilizer or CO2 to your tank if necessary so that the plants have all the nutrients they need to remain healthy and grow.

Clogged filters

No matter its size, every fish tank should have an efficient filtration system.

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

As the water passes through the filtration system, any floating particles of waste and debris are removed by the mechanical filter. Eventually, all that debris accumulates in the filter media and inside the filter box as thick, smelly sludge, and that can make your fish tank really stink.

Also, once the filter media gets clogged with sludge and muck, the water can’t circulate through the system properly. That means that the beneficial bacteria in the biological filter media can’t do their job properly, and the quality of the water in the tank will suffer, which is very bad news for your fish.

Preventing foul odors caused by your fish tank filters

The way to prevent your filter from becoming stinky is to clean it regularly.

That’s a simple job 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’ve removed when carrying out a partial water change. That will get rid of any accumulated sludge and gunk. The biological filter media doesn’t need washing unless it has also become clogged with gunge.

If you have especially dirty fish or a densely populated tank, you may need to clean the filters more frequently, say once every three weeks or so.

Most filter media has a shelf life, so you will need to replace the filter cartridges or refresh the media, depending on what kind of filtration system you have. Check the manufacturer’s instructions for your filter to find out how often you need to replace the filter media or cartridges.

Dirty substrate

If the stench coming from your aquarium smells like rotten eggs, that’s sulfur. But how can that happen, and where does the sulfur come from?

So, fine gravel or sand becomes compacted over time when placed at the bottom of a fish tank full of water. Within the substrate, small air pockets form, trapping waste. Bacteria feed on the waste, and it’s those bacteria that are responsible for that revolting smell, as the gas they create rises up through the substrate and water into the air.

Often those dead zones go undetected until you move the substrate, perhaps when planting new plants or moving tank decorations around. Once the substrate is disturbed, the air pocket is ruptured, and the gas escapes.

Preventing dead zones from forming in your substrate

Again, preventing dead zones from forming in the substrate is down to good tank maintenance practices.

While carrying out a partial water change every week, use an aquarium vacuum to agitate the sand or gravel throughout the tank. As you go, make sure that you cover all areas of the bottom of the tank, paying particular attention to the corners, around the base of plants, and underneath decorations.

Water conditioner

Although you must use a water conditioner to remove toxic chlorine or chloramine from tap water before you add it to your aquarium, some conditioners can make the water in your tank smell a little “off.”

However, that eggy aroma that’s caused by sulfur in the conditioner formula generally disappears once the conditioner has dissipated throughout the tank.

How to freshen up your aquarium … and keep it smelling fresh!

Now that you’ve identified the cause of the whiffy water in your aquarium and removed it, you want to freshen up your fish tank and keep it that way!

Remove any decomposing organic matter

Your first job is to get rid of every dead and decaying thing in your fish tank, including:

  • dead livestock
  • dead plant matter
  • fish waste
  • dirty filters

Start by carrying out an inventory of all your fish, shrimp, snails, and other creatures you have living in your tank. If anyone is missing, search the tank thoroughly, checking all the usual hiding places to make sure that there are no unseen corpses decaying there. If you find a deceased animal, remove it immediately.

If all your fish are present and correct, move on to check your live plants. Trim off any dead leaves or broken stems, and remove dropped leaves that have fallen behind decorations or are trapped between the plant stems at the bottom of the tank.

Clean your tank thoroughly

  1. Use an algae magnet to clean the aquarium viewing panes thoroughly.
  2. Next, use an aquarium vacuum or siphon to give the substrate a thorough deep-clean and remove fish waste, uneaten food, and plant debris that’s trapped there. Pay close attention to corners where dead zones can form.
  3. Remove tank decorations and clean them thoroughly, scrubbing away algae and general detritus.
  4. Now take out the filtration system and clean it thoroughly. Wash the mechanical filter media in tank water to get rid of any sludge that could be preventing a good flow through the system. If necessary, discard and replace spent filter cartridges or filter media that’s past its best.
  5. However, don’t replace all the biological filter media at once. If necessary, replace one third at a time so that you don’t cause the biological filter system to crash and kick off a fresh nitrogen cycle.
  6. Be sure to clean the filter outlet and inflow, and check that the impeller is free from gunk and able to rotate properly.
  7. Carry out a 20% water change.

It still stinks!

If your fish tank was really dirty, it will almost certainly still smell, even after you’ve carried out all the above-listed maintenance tasks.

That’s because you most likely stirred up a whole lot of muck that’s now floating around the water. So, over the next few days, carry out a 10% water change every day to gradually refresh the water.

The water should now appear clearer, and the smell should have pretty much gone. However, to be on the safe side, we recommend that you check the filter media again. If the water was full of floating particles of debris after the initial deep cleaning exercise, there’s a good chance that the mechanical filter will need washing through again.

What about a carbon filter?

Many hobbyists swear by activated carbon as a highly efficient way of removing odors from the tank. Activated carbon filters also work to keep the water clear and free from discoloration. Your filtration system may already incorporate carbon filters within the system, but if not, there’s a chance you can customize the unit to do so.

The main drawback to using carbon filters is that they only last for a month or so before you need to replace them. That’s not a problem if you only want to use the filter on a temporary basis to remove the bad smell from your tank after cleaning it. Also, the carbon removes most fish medications from the water, too, so if you want to treat any of your fish, you’ll need to move them to a quarantine tank for the duration of their treatment.

Final thoughts

So, the bottom line is: if your fish tank smells, that’s most likely down to poor maintenance and management.

If you keep your aquarium clean, maintain the filtration system correctly, and remove any dead organic matter as soon as you see it, your tank’s healthy aroma should remain undetected until you lift the lid and take a deep breath.

Use the tips and information we’ve provided in this guide to keep your aquarium spotless, and you’ll enjoy the sweet smell of success instead of the foul stench of decay!

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