Is Mold Bad for Fish Tanks

Is Mold Bad for Fish Tanks? Read This First!

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Fish tanks are known for housing a lot of unique creatures as well as growing a lot of sometimes unusual organisms and substances. A common growth like algae is something that is natural and oftentimes easily managed depending upon the tank’s fish species and filtration needs being met, but occasionally aquarium owners will find their tanks growing mold in and around the water as well.

Is mold bad for fish tanks, though?

Yes, mold is terrible for fish tanks and something that needs to be addressed immediately. Fish and plants inside can succumb to a host of diseases and infections that you may not be able to treat effectively whenever mold is present. If you notice it growing in your tank in excess, you’ll need to clean it as soon as possible.

While it isn’t good for a tank to have mold, it’s nothing to be panicking about if you see only one tiny speck—that is, of course, unless you see your fish and plants suffering because of it. Mold is a very normal and common occurrence in a tank, but when it gets out of hand, it can create some significant problems.

What Causes Mold in Fish Tanks?

There are many reasons why mold develops in a fish tank. While there is a world of causes for a fish tank to have mold, the ones mentioned below are some of the most common culprits. Unclean water or an imbalance of nutrients is at the top of the list.

Excess Food

Unfortunately, it’s far too common for fish keepers to overfeed their fish. This is the number one cause of moldy tanks because the extra food will produce phosphorus fumes. The excessive abundance of phosphorus provides the breeding ground for mold to appear, and the resulting chemical reaction can become very dangerous for fish.


Some decorations look fabulous in a tank, but the materials these items are made of may not be healthy for the delicate balance in your mini ecosystem. Watch your plants for any signs of struggle. If they die, oxygenation slows down, and this provides mold with exactly what it needs to fester.

Poor Cleaning

If a tank isn’t cleaned regularly, ammonia and nitrates build up. These are very toxic to fish, and they are the cause of brown or black mold developing.

When your tank becomes cloudy, you need to be sure to clean it as soon as possible. This cloudiness is caused by food, waste, and other debris in the water. Unfortunately, this dirty environment provides the perfect environment for nasty molds to appear.

Too Much Light

Fish tanks sitting in direct sunlight provide some of the conditions for mold to take over the tank. This is due to the temperature fluctuations causing chemical changes in the water. This issue becomes most noticeable at the top corners of the tank, so be sure to keep your aquarium located away from sunlight and use LED lights to provide controlled lighting for your setup instead.

How Do You Get Mold Out of a Fish Tank?

When you have mold that you need to get rid of, you will have to devise a plan of action to attack the problem. This may be something of a time-consuming process, so set aside a chunk of your schedule to make sure you are well-prepared and ready to resolve the problem. Make sure you can dedicate yourself to ensuring the tank is clean and free of mold.

Items You’ll Need:

  • Siphon
  • Windshield/Glass Scraper
  • Chemical-Free Paper Toweling
  • Clean, Unused Sponge
  • Hot Water
  • Bucket or Quarantine Tank
  • Another Bucket
  • Water Clarifier/Dechlorinator
  • Filter Replacement Cartridge
  • Clean, Unused Toothbrush
  • Hot Water
  • White Vinegar
  • Clean, Prepared Aquarium Water


Before going through these steps, understand that you may have to add more or remove some of the listed items depending upon your situation. Therefore, consider each step strategically when devising your plan.

  1. Remove the Fish: Take the fish out, and put them in a bucket or quarantine tank with prepared tank water.
  2. Siphon the Water: Take out the water with a siphon designed for an aquarium. If you can, try to retain about 10% of the water to provide healthy bacteria. However, if you have to empty the entire tank of its water to ensure a proper clean, please do so. Be sure to dispose of the water safely, either down the drain or at a waste facility.
  3. Remove Items: Take out the décor, substrate, filters, water pumps, and any other items. Take apart the filter, and put all of the items included with it in a bucket with prepared aquarium water. Clean them with a toothbrush or clean sponge, and don’t use or rinse them with tap water.
  4. Clean the Substrate: Use a fine mesh strainer or colander to rinse off the substrate in the bucket of prepared and clean aquarium water. Use your hands to scrub all the mold off, and then scrub any additional concerns with the toothbrush, if needed.
  5. Clean the Lid: Mix a half cup each of white vinegar and hot water in a clean spray bottle. Shake well and then spray this mixture generously onto the inside of the lid. Let this sit for about five minutes, then use a dry sponge to wipe it all down. Rinse off your sponge only in the bucket of prepared aquarium water.
  6. Clean the Tank: Use the windshield/glass scraper to remove any mold stuck to the sides of the tank. Wipe the sides with chemical-free paper towels. Don’t use cleaning detergents or even the vinegar mixture on these parts of the tank—just the scraper and the paper towels.
  7. Reset the Tank: Put any pumps and filters back together, ensuring you replace the filter cartridge. Put the substrate and decorations back in. Refill the tank with the prepared aquarium water, and use your water clarifier and dechlorinator.
  8. Put Aquatic Life Back Home: Return fish, plants, and other aquatic life back into the tank. Be careful and make sure all parameters remain the same as they were before cleaning to reduce the amount of shock as much as possible.


The best way to control mold buildup in fish tanks is by staying on top of regular cleanings and partial water changes. However, when it does take hold, you’ll need to clean it immediately as it is guaranteed to destroy your fish. Depending upon how bad it is, you may have to clean the entire tank or only a small portion of it.

See Also:
Will Magic Eraser Kill Fish? (A Detailed Explanation)
How Long Does It Take for Dry Rock to Become Live?
Can You Put Aloe Vera in a Fish Tank?
Can Clownfish Live Alone? (A Detailed Explanation)
Does a Fish Aquarium Lid Need Holes?