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Fish Tank Mold: Removing Unwanted Fungus From Your Aquarium

No one likes seeing a cloudy fish tank. Routine cleaning and hygiene should prevent such a sight. But sometimes that green film covering everything from the sides of the tank to the décor? It isn’t what you think.

Mold can find its way into a fish tank with little trouble. And removing mold – especially once it’s grown out of control – is an undertaking. But fish tank mold is harmful – to your fish AND you.

Fish Tank Mold

People don’t often think of mold as popping up in fish tanks. On ancient leftovers in the refrigerator, sure. But in a fish tank? That seems strange.

But why? Mold is nothing more than a fungus. And fungi only need a few things to grow:

  • Moisture
  • Light
  • Sustenance

You can find all of those things in a fish tank. That goes double when the cleaning schedule’s fallen behind, or fish have received too much food. Waste builds up in the system, giving mold a chance to spread. And in a fish tank, you have the unique condition of underwater AND airborne fungus in one handy location.

Recognizing Mold in a Tank

Recognizing Mold in a Tank

Aquarists may overlook mold in their fish tank, especially green mold. The fungus looks similar to algae. (Not that algae are a favorite to have around in a tank, either) But you’ll notice subtle differences emerge as the mold makes itself at home.

That green film doesn’t confine itself to the tank sides. It slowly spreads over the glass, the substrate and even attaches itself to your fish. You’ll notice the mold below AND above the water clinging to the underside of your aquarium lid. It often takes on a fuzzy texture – precisely like the mold you wrinkle your nose at when you open the fridge.

And if it’s crawling across the lid? You’re already breathing in the spores it’s sending out into the room. (Something fun to contemplate)

Mold on tank lid (Image courtesy of Tim Regan from Flickr)

Consequences of Mold

Mold, on its own, is unattractive in a fish tank. It covers the glass, obscuring the view. And all of your décor? It starts to look ugly.

But fish tank mold is also harmful. If you fail to address the fungus spreading throughout the aquarium, your fish can end up with several infections. Mold can attach to fish and invertebrates. As they’re already under stress from the poor water quality, they have lower immunity. And the result?

  • Cotton wool disease
  • Fin rot
  • Gill rot
  • Mouth rot

As the fungal infections continue, secondary diseases go to work. And before long, your fish begin to die.

Removing Fish Tank Mold

Preventing fish tank mold from appearing is the best course of action. But it’s also a natural part of the aquatic world. This means you’re going to see mold now and then. So it’s essential to understand how to remove mold as soon as you spot it.

And this goes double if you purchased an aquarium from someone else. Mold spores can happily lay dormant on glass, tucked away in the corners. Before you start to set up your tank, follow the same steps you would for mold removal. It’ll save you pain and heartbreak down the road.

Supplies for Mold Removal

Fish tank removal requires some essential supplies. Before you start, make sure you have all of the following:

  • Vinegar
  • Hot water
  • Sponges
  • Toothbrushes (preferably not one you’re currently using)
  • Paper towels
  • Siphon
  • Glass scraper
  • Bucket
  • Colander

Safety First

Mold – even fish tank mold – is harmful to humans. It can lead to respiratory problems. So whenever you’re dealing with a fish tank mold removal process, you must wear a mask.

This will protect you during the cleaning. You’ll also want to wash your hands as often as possible. (If you touch anything you suspect is mold? Reach for the soap) And don’t wear clothes you’re attached to. Remember, mold spores are sneaky, and they like to attach themselves ANYWHERE.

Keep fish separate for their safety (Image Courtesy of Dennis Sylvester Hurd from Flickr)

Your Fish

Fish tank mold removal is a brutal undertaking. You may need to remove over 50% of the water from the tank – and that’s for a MILD problem. (For severe infestations, the entire tank gets drained) As such, you don’t want to shock your fish. You’ll need a separate container with CLEAN water. Make sure it matches the tank’s water conditions. It’s also a good idea to keep it in a different room from where you’re cleaning.

Step 1: The Lid

Fish tank mold has a weakness: vinegar. So you’re going to want to prepare a solution of one part vinegar to one part (clean) water. Putting the solution in a spray bottle will make it easier to work with.

  1. Remove the lid and set it on a surface covered with paper towels.
  2. Spray the lid, scrubbing the mold (You can use a sponge or toothbrush).
  3. Rinse the lid with hot water.
  4. Spray the lid with the vinegar solution again. This time, let it sit for 2 minutes.
  5. Repeat the scrubbing process.
  6. Rinse with hot water until you can no longer smell the vinegar.
  7. Leave to air-dry.

Step 2: The Tank

Vinegar is fine for the OUTSIDE of the aquarium, but you don’t want it on the inside. It’s acidic and will drop your water’s pH too low. Be careful with that spray bottle.

  1. Drain the water with your siphon. If the fish tank mold’s bad, empty the entire tank.
  2. Empty the water OUTSIDE and as far from the house as possible.
  3. Scrub the glass with your grass scraper (and lots of elbow grease).
  4. If you need, break out a fresh toothbrush to get into those corners. (Remember, no vinegar goes inside)
  5. Rinse the tank with hot water. Continue until the siphon runs clear.
  6. Rinse out your filter with hot water, throwing away the old cartridge.
  7. Spray the OUTSIDE of the tank with the vinegar solution and wipe it down.
  8. Leave the tank to air-dry.

Step 3: Décor

Rocks, gravel, and plants will return to the tank once clean. This means you can’t use vinegar. (And vinegar will kill live plants) Try to separate your substrate from the rest of the decorative pieces.

  1. Submerge everything in hot water. (If you can manage it, use boiling water) Leave it for 5 minutes.
  2. CAREFULLY (no burns), scrub everything with a toothbrush that HASN’T touched vinegar. Gravel can get stirred around to loosen mold particles.
  3. Rinse everything with fresh hot water and the colander.
  4. Continue the scrubbing and rinsing until the water runs clear.
  5. If you have items that can survive the process, boil them for 2 minutes as an extra sterilization process.
  6. Spread everything on CLEAN paper towels to air-dry.

Step 4: Revisiting the Tank

Mold creates spores, and it clings to glass and corners. You’ll want to give your aquarium a second thorough cleaning before you start to reassemble everything.

  1. Break out the glass scraper one more time to ensure you’ve removed EVERY bit of mold.
  2. Use the toothbrush to reach those pesky corners.
  3. Rinse down the tank with hot water, making sure the siphon runs clear.
  4. Set up a new filter cartridge in your system.
  5. Return the substrate and décor to the tank.
  6. Fill the tank with CLEAN water. Make sure you use your usual dechlorinator and water clarifiers.
  7. Test your water quality.
  8. Gently return your fish to their mold-free tank.

Step 5: Clean-Up

There’s a good chance you have mold spores attached to the tools you scrubbed with. With UNDILUTED vinegar, rinse everything thoroughly. Then rinse them again with hot water. If the water looks cloudy, repeat the process until it rinses clean.

For the toothbrushes and sponges, consider throwing them away and buying replacements. They have crevices and bristles where spores may hide. And you don’t want to risk introducing mold to your fish tank the next time you clean.

Preventing Fish Tank Mold

Planted tanks need proper rotation to prevent decay (Image courtesy of Amuljar from Pixabay)

Now that you have a sleek, shiny, mold-free fish tank, you want to keep things that way. So how do you keep mold out of your aquarium? Mold grows where there’s excess waste in a system. This means tanks with poor water quality, untended filters, and overfeeding. But other problems also contribute to mold problems in a fish tank:

  • Plants: Live plants aid the nitrogen cycle. They also supplement the diet of some fish. But they’re challenging to manage. If you don’t replace your plants every few weeks, they decay. That feeds directly into mold growth.
  • Light: One of the critical components for fish tank mold is light. And sunlight? It’s the best. Fish tanks placed close to direct sunlight often struggle with outbreaks of mold. You’ll see the fungus starting in the corners of the tank closest to the sun and moving down. You want to avoid natural sunlight as much as possible.

If you want to cut down on the risks of mold, you need to keep your aquarium clean at ALL times. This means performing weekly water changes. SOME fungus in the water is beneficial to the nitrogen cycle. The fungal cells work with the bacteria to keep your fish and plants healthy. But you don’t want things to slide out of control.

You should also only feed your fish the amount of food they can eat within FIFTEEN MINUTES. If you see leftovers hanging around, it’s time to cut back. The excess particles of food make the water cloudy and feed mold growth. Plus, those leftovers are your fish letting you know they’re full.

Frequently Asked Questions About Fish Tank Mold

Are There Different Kinds of Fish Tank Mold?

You tend to see three types of mold in a fish tank:

  • Green mold: This green film is often mistaken for algae in the average fish tank.
  • White mold: White fungus can start green before turning brown and then changing to a white color. It leads to the most health problems in fish.
  • Black mold: Black mold is rare, but it thrives when you have excess decomposition present.

All of them arise from poor water quality and too much waste in the aquarium environment. And cleaning them? You use the same process.

Is Mold the Same as Algae?

Mold and algae come from different families. And while they use the same resources, they don’t behave the same. No one enjoys fighting algae blooms in their fish tank, but you have plenty of resources available to combat the problem:

  • Algae-eaters
  • LED lights designed to limit algae growth
  • Nitrate removers

When it comes to mold, those solutions aren’t effective. Though, of course, managing your cleaning schedule and limiting the waste you allow will help with algae OR fish tank mold.

Should I Change My Tank’s Filter?

If you’re finding mold in your fish tank – especially after performing a thorough cleaning, you need to consider a filter change. The power in your filter may not be strong enough to cope with the stock in your tank.

Remember, the filter needs to handle EVERYTHING in your tank: fish, plants, and invertebrates. And while the power listed may work for your tank size, it may end up struggling to keep up with the bioload. Canister filters have the most power. So if you see fish tank mold, you may want to consider an upgrade.

Can I Use Cleaning Products?

The problem with cleaning products in mold removal is the hazard they pose to your fish and other tank residents. Even with thorough rinses, residue can hang around. The chemicals in cleaning products AREN’T approved for fish. Even the diluted vinegar is too strong and can lead to problems with the water values.

You’re better off sticking to hot water and elbow grease. You’ll keep your fish safe. It requires more work, but at least you won’t risk fish getting sick or dying as a result.

Unwanted Fungus

Every cycled fish tank contains SOME fungal components. But fish tank mold? That’s disgusting, and no one likes seeing it creep over the glass sides and lid.

You need to look over your maintenance to see if you can make improvements. And then you can perform a thorough fish tank mold removal. With some work, a clear, shiny, mold-free aquarium IS possible.

And your fish will remain safe and healthy – with no fuzzy fungus invaders.


  • Aquarium Science – link
  • MoldBlogger – link
Jason Roberts
About Jason Roberts
Jason is an aquarium fanatic that has been a fish hobbyist for almost three decades.

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