Why is my fish tank green?

A green fish tank is no need to fret! Your fish tank could be green for a variety of reasons. Most of these reasons are easy-to-fix and you can have your green fish tank clear again in no-time!

What makes my fish tank green?


Algae is a common occurance in fish tanks. Algae can slowly take over regions of your tank or in some cases the tank can be covered entirely. This is a common cause of green fish tanks.

Inadequate filter/circulation set-up

Another common cause of a green fish tank can be issues with the cycle of the fish tank. Overfeeding can result in excess nutrients in your tank which can cause algae. Additionally, the food particles themselves can cloud the water making your fish tank green. Without adequate filtration and circulation, this matter will cloud the water.

How can I clean a green fish tank?

While further down this page we discuss ways to prevent a green fish tank in the future, here we will discuss how to immediately clear your green fish tank.

Identify the root of the problem – If this is algae, is it caused by overfeeding, overlighting, or perhaps something else? If your problem was an inadequate tank cycle (filter or circulation) identify if the problem is caused by inadequate filtration or inadequate flow.

Clean the Green Fish Tank – With a set of tools you can find at a local pet store, you can clean the fish tank. Additionally you can check us out and see if service is available in your area!

  • Be careful not to scrape any living organisms.
  • Clean any glass or acrylic surfaces with a sponge or brush.
  • Metal scrapers must not be used on acrylic surfaces
  • With a gentle brush, clean the live rock surfaces and any nooks and crannies the brush/sponge couldn’t get.
  • Vacuum the gravel/sand from the bottom of the tank, removing any debris or detritus
  • Replace with RO water
  • Clean filters and tubing 72+ hours later.

**Do not clean your fish tank and assume the problems are gone

Looking for an alternative?

Check out our Who we Serve page to see if Son’s Fish aquarium services are available near you!

Make a plan of action – Once your fish tank is no longer green and you have identified the root of the problem, you can make a plan to prevent a green fish tank in the future.

  • Overfeeding Research your specific species of fish and make sure they do not require special diet considerations. Most fish can safely go 2-3 days a week without being fed, so long as the days are not consecutive.
    • Skip feeding your fish one day a week, see if your fish tank continues to turn green
    • If your fish species does not require special feeding schedules, skip feeding another day a week that is not consecutive with the first skipped day.
      • Skipping Monday and Thursday is okay. Do not skip two days in a row like Saturday and Sunday.
  • Inadequate Flow/ Filtration – If a reduction in feeding hasn’t helped or isn’t possible, you must identify if your fish tank is green because of lack of flow, or because of a lack of proper filtration.
    • Look at your water – Can you see visible chunks of matter floating around?
      • If this is the case – consider adding a protein skimmer to your fish tank. This will help capture extra food particles and waste generated by higher amounts of feeding or activity.
    • Did your green fish tank look more cloudy but still fairly clear?
      • If this is the case – consider adding a filtration medium like GFO (Granulated Ferric Oxide – BulkReefSupply Instructions Here) or activated carbon. NOTE: These media are not added directly the water, they are added in a filter media bag.

Staying on top of a green fish tank in the future

After your fish tank is clean there are several ways to keep things green-free in the future. Some of these methods were mentioned previously as they may have helped in the fish tank cleaning process.

  1. Granulated Ferric Oxide
    • Removes phosphates – this inhibits algae growth
  2. Activated Carbon
    • Removes inorganic and organic materials dissolved in water – Lowering excess nutrient levels
  3. Refugium
    • For larger fish tanks – Serves many purposes including nutrient export, denitrification and circulation
  4. Cleaning algae as soon as you see it
  5. Rinsing filters and performing water changes regularly.

Algae eating fish and critters

Here is a a quick (by no means all inclusive) list of Algea eating fish or critters you can put in your green fish tank. Be careful when adding a large amount of any of these creatures as they can and will eat all of the algae and starve unless they have another food source.



  • Bristlenose Plecostomus (Bristlenose plecos)
  • Siamese Algae Eater (Flying Fox)
  • Chinese Algae Eater
  • Otocinclus Catfish
  • Twig Catfish
  • Live-bearers
  • Mollies
  • Whiptail catfish


  • Mystery Snail
  • Nerite Snail
  • Malaysian Trumpet Snail
  • Ramshorn Snail
  • Apple Snails
  • Red-rimmed melania
  • Anentome Helena
  • Rabbit Snail


  • Cherry Shrimp
  • Amano Shrimp
  • Cardinal Shrimp
  • Ghost Shrimp


  • Tailspot Blenny
  • Kole Tang
  • Foxface
  • Black Combtooth Benny
  • Highfin Benny
  • Blonde Naso Tang
  • Pacific Blue Tang
  • Red Sea Sailfin Tang
  • Yellow Tang
  • Goldrim Tang


  • Nassarius Snail
  • Cerith Snail
  • Mexican Turbo Snail
  • Chestnut Cowries
  • Margarita Snail
  • Banded Trochus Snails
  • Zebra Turbo Snails


  • Emerald Crab
  • Dwarf Red Tip Hermit Crab
  • Blood Red Fire Shrimp
  • Scarlet Skunk Cleaner Shrimp
  • Electric Blue Hermit Crab
  • Urchins

NOTE: Pease make sure each species is compatible with your fish tanks ecosystem and current conditions. Son’s Fish is not liable for any damages due to fish tank neglect.

Why is my fish tank green?
%d bloggers like this: