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How to Clean Mold from a Fish Tank

by Amanda Demsky
mold in fish tank

Is Your Fish Tank Growing Mold?

The health of fish and their owners depend on tank hygiene. This is because every fish tank provides the perfect environment for both submerged and airborne forms of fungi. General tank cleanliness, water quality, filtration, decor material, and the initial health of the fish all contribute to the growth and dispersing of mold spores. A dirty tank allows a film to build both underwater and in contact with the air above. Decomposing organic material, such as wood decor, dead fish, excrement, or the open, seeping wound (infection or injury) of a fish can all provide nourishment to underwater fungus, as well as airborne mold.

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If conditions are favorable, the first sign may be a visible film on the glass, decor, rocks, or even on the fish themselves. This film may appear green and slimy (sometimes even red), or it may have a fuzzy white, gray, or brown appearance. If such a film is visible, not only are the fish at risk, but there is bound to be airborne mold attached and growing somewhere on the tank, as well. Generally, if airborne mold is present, it will be discovered on the inside of the lid and open-air portions of the glass. This mold is what can and will directly affect the health of the owner. It can appear in a variety of colors and textures, but the most common tank mold will appear brown or even black with a fuzzy texture.

When fungi does not appear to be present, the general rule to cleaning a tank is to leave all the items (even the fish) inside the tank and replace only 10-15% of the water. This is to ensure that any beneficial bacteria and fungi is left unharmed, as they will act as a natural filter for the tank. However, if the condition of the tank’s water seems compromised by fungi, it will be necessary to remove the fish and all decor, including gravel/pebbles, and at least 50% of the tank’s water should be replaced. MoldBlogger suggests removing 100% of the water and starting fresh if fungus is suspected.


Items Needed:

  • fish net
  • aquarium siphon
  • 1 separate container large enough for fish
  • 1 (maybe 2) separate container(s) (1 can be a colander) large enough for tank decor and safe for high temperatures
  • 1 never-used toothbrush
  • algae/glass scraper
  • sponge (optional, but helpful in covering a larger surface area)
  • all-natural, chemical-free paper towels (optional)
  • a replacement filter cartridge
  • dechlorinating treatment
  • water clarifier
  • hot or boiling water
  • non-tap water as replacement in the tank
  • undiluted white or apple cider vinegar in a spray bottle or bowl
  • mask or covering to avoid inhalation of mold spores (optional: at your own risk)


The #1 Rule:

Do not use any type of cleaning product on your fish tank or on any of its contents. Trace amounts of the cleaner’s chemicals could remain after rinsing and cause harm to your fish.


Step 1 & 2: Tank Lid & Fish

  • Put on the mask.
  • Take the lid to another surface away from the tank and tank contents.
  • Wash hands thoroughly.
  • Fill a separate container with water (not from tap) and place the fish in there. (Be sure the fish are nowhere near the area you have chosen to clean the lid in.)

Vinegar can lower the pH of the water and harm your fish, which is why it is only permitted as a cleaner on the outside of the tank and on the inner lid–as long as it is adequately rinsed.

  • Spray the lid with undiluted vinegar and wipe clean (sponge, toothbrush, paper towel). (Or apply vinegar by saturating the sponge or paper towel with it from a bowl.)
  • Rinse with hot water and apply more vinegar. Let sit for 2 minutes.
  • Scrub lid again and then rinse with hot (or boiling) water.
  • Repeat the hot water rinse until any remaining scent of vinegar has dissipated.
  • Let sit to air-dry until all the other steps are complete.


Step 3: The Tank

  • Drain the tank water with the aquarium siphon (follow instructions from siphon packaging) into another container. Leave 50% in the tank if infestation is mild. Otherwise, siphon it all and toss it outside a good distance from your home.

Step 4: The Decor

  • Remove all decor, including gravel/pebbles and place them in a large container safe for high temperatures. (Separate the larger decor from the gravel/pebbles in two different containers, if desired. A colander can be used for large gravel/pebbles.)
  • Pour hot water (boiling, if preferred) over the items until they are fully submerged. Let sit 5 minutes. (If a colander is used, pour hot water slowly over all contents as a rinse.)
  • Scrub away any remaining residue with the toothbrush. (The gravel/pebbles can be stirred around by the toothbrush to dislodge fungal particles.)
  • Rinse the items with hot water. (Repeat this process with freshly-hot water again if the items seemed particularly infested.)

Step 5: The Tank Again

  • Use the algae/glass scraper to scrape any mold or algae from the aquarium’s inner glass.
  • Scrub the glass with the toothbrush (or sponge) using hot water.
  • Rinse the sides of the tank with hot water and siphon out the results.
  • Optional: An all-natural, chemical-free paper towel can be used to wipe down the sides and bottom. (Never use a paper towel on the inner tank or lid that is not guaranteed to be natural and chemical-free.)
  • Replace the filter cartridge and thoroughly rinse the filter with hot water.
  • Situate the tank’s decor back in place.
  • Refill the tank with water (not tap) and add the dechlorinating treatment and water clarifier. (Follow the instructions located on each item’s package. Sometimes, both can be purchased in a water treatment kit.)
  • Wash hands thoroughly.
  • Gently return the fish to their newly cleaned tank. Say, “You’re welcome.”

Water Treatment: The dechlorinating treatment protects your fish from any traces of chlorine in the replacement water. The water clarifier dissolves excess substances that contribute to fungal overgrowth and harmful bacteria.

 Step 6: Cleaning Items & Containers

  • Toss all cleaning items (sponge, toothbrush, used paper towels, and even the mask) in the trash and take directly to the bin outside.
  • Thoroughly wash all containers used, rinse with undiluted vinegar, rinse again with hot water.
  • Thoroughly wash your hands.
  • Share this article on social media and congratulate yourself on being such a fine fish owner.

Maintenance Tips:

  • Overfeeding contributes to food particles accumulating throughout the tank, contributing to cloudy water and the overgrowth of fungi and bacteria. Make an effort to feed your fish only as much food as they can eat in 15 minutes or less to reduce this effect.
  • Change 10% of the tank’s water each week to prevent further fungal growth.
  • Repeat the steps for washing the lid once a month, making sure to clean it far from the tank/fish and to rinse thoroughly with hot water before replacing.


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For more information regarding mold, mold prevention, and mold solutions, please check out the rest of MoldBlogger.com.


About the Author: TheWife is the mother and personal chef of two boys, the domestic technician of a three-bedroom desert home, and occasionally, a freelance writer and editor. Feel free to follow her on Twitter @TheWifesLife

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1 comment

Jack Dempsey July 7, 2018 - 6:21 pm

I’ve never thought to use vinegar to clean things, just used extra elbow grease. One question how can you tell the difference between mold and algae?


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