How to Clean Mold in a Fish Tank

While mold can grow on the side of a fish aquarium, "mold" on submerged tank decorations or floating in the water itself is more typically algae or fungus. You can quickly remove these substances with a thorough cleaning, but must resolve any water chemistry problems to keep mold from returning.

Drain the water from your fish tank into a bucket. You will need to replace all the water in the tank to remove mold and prevent it from returning. Place your fish in a second tank or in a bucket of water from the aquarium—not tap water—before draining.

Remove any ornaments and decorations from the aquarium and rinse them in hot water for five minutes. Scrub them with a toothbrush to remove any residue. (Do not use any type of cleaning product on your fish tank or anything that goes in it; trace amounts may remain after rinsing and harm your fish.)

Scrape any mold, algae or other residue from the aquarium glass with a glass scraper. Then scrub the glass with a sponge or toothbrush and hot water.

Replace the filter cartridge in your tank's filter and thoroughly rinse the filter itself in hot water.

Refill the tank with water and add a dechlorinating treatment and a water clarifier. The dechlorinator will keep your fish safe, while the water clarifier can help dissolve excess substances that contribute to the overgrowth of mold and other organisms. Follow the directions on the water treatment's package.

Items you will need

  • Aquarium siphon
  • Toothbrush
  • Glass scraper
  • Water filter cartridge
  • Water treatment


  • Feed your fish as much food as they can eat in 15 minutes, but no more. Overfeeding can contribute to cloudy water and the overgrowth of mold and other organisms.
  • Change 10 percent of the water in your tank each week to prevent mold regrowth.

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About the Author

Brenna Davis is a professional writer who covers parenting, pets, health and legal topics. Her articles have appeared in a variety of newspapers and magazines as well as on websites. She is a court-appointed special advocate and is certified in crisis counseling and child and infant nutrition. She holds degrees in developmental psychology and philosophy from Georgia State University.