How to kill green hair algae in a saltwater aquarium

how to kill green hair algae with hydrogen peroxide

Getting rid of green hair algae in a saltwater tank brings enough stress to drive you crazy. Green Hair Algae in a saltwater aquarium is incredibly difficult to get rid of and will make you want to quit the saltwater aquarium hobby. I have a 125 gallon saltwater tank and host the YouTube channel Rotter Tube Reef.

After trying everything to get rid of green hair algae in my saltwater tank, I found something that truly works and the green hair algae never came back! People said cover your aquarium so no light hits it, use phosphate rx, feed your fish less, etc etc and NOTHING WORKED! Hydrogen Peroxide dosing worked and the green hair algae was gone for good with very little effort.

Food grade hydrogen peroxide, at the 12% concentration level, dosed at 3ml per 10 gallons of aquarium water, did the trick! This is the hydrogen peroxide I used but any 12% rated hydrogen peroxide should work fine for you. It did not harm my inverts or corals. Corals did close up a bit for a few days but came back with no issues.

I could write a very long post on green hair algae, commonly referred to as GHA, but I will not. The purpose of this post is to let you know what I did to kill off all the incredibly nasty green hair algae in my saltwater tank in less than a week.

Green Hair Algae gets its name because it looks like silky green hair. It will start off in your aquarium as flat green spots on the rock. If you let it grow, it will get out of control fast, resembling hair, which is connected to your glass tank walls, rock and anything it can attach to. Once Green Hair Algae gets a foothold in a saltwater aquarium, it can soon cover everything in your tank if prompt measures are not taken.

how to kill green hair algae with hydrogen peroxide
green hair algae spreads like cancer in a saltwater tank

The cure for Green Hair Algae is the same as the prevention: Starve it into oblivion. Green Hair Algae require not only light but also nitrates and phosphates in order to survive. Nitrates can be introduced into an aquarium not only as the end product of the Nitrogen Cycling Process (ammonia/nitrite/nitrate) but also via some brands of commercial sea salts and tap water.

Phosphate (PO4) is a part of life on earth. Virtually every living thing contains some phosphates and they can enter the tank in a number of ways. Fish & critter foods, tap water, and carbon are some of the phosphate generators in your tank. This is why it is important to NOT OVERFEED YOUR FISH!

There are many fixes on the internet for getting rid of green hair algae. Being that this evil feeds off sunlight, it is strongly advised to not place your aquarium by a window, when initially setting it up. Although this is good advice, my tank has been by a window for years with no issue.

You need to have only PURE WATER when making saltwater for your aquarium. For this reason, find a reef store that has PURE saltwater or make your own at home with the use of an RODI walter filter system. NEVER use tap water when making saltwater or topping off your tank from evaporation.

People have done all types of things to rid their tanks of GHA and I’m one of them. Although some of these products are great, they will not work in all aquariums. Some items to try include phosphate RX, fluconazole for fish, seachem phosguard, red sea nopox and others.

What worked best for me is dosing the tank with 35% food grade hydrogen peroxide. 12% hydrogen peroxide works just as well, using 2ml of hydrogen peroxide per 10 gallons of aquarium water. The GHA in my tank was out of control and hydrogen peroxide worked perfect. I dosed 2ml for every 10 gallons every other day for a week. For best results, turn off all pumps so your water is very still. Get a syringe to measure the amount and gently dose over a rock with green hair algae. Leave pumps off for 10 minutes then power up. Do this to different parts of your tank until all is gone. The rest of the hydrogen peroxide will flow through the entire water column in your aquarium, dissolving other green hair algae.

The removal of green hair algae will drive you crazy. Check out this video I made that shows how I killed it with hydrogen peroxide. It never came back.


Make sure not to get the hydrogen peroxide on your skin, as it will irritate it. Use gloves and practice caution. This hydrogen peroxide will not harm fish, corals or invertebrates. Corals may close up for a couple of days, which is normal.

I have seen the 35% hydrogen peroxide show up as not available at times. The biggest reason for this is it’s hard to keep in stock. If this is the case and you want it now, the 12% will work well and you can dose double the amount. My experience is with the 35% and it works perfectly. Green hair algae does not return.

If you have any questions let me know. I spent weeks researching and most things did not work on my disgusting tank. 35% food grade hydrogen peroxide did the trick and turned my tank from nasty to crystal clear.

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5 thoughts on “How to kill green hair algae in a saltwater aquarium

  1. H2O2 is amazing stuff in the aquarium hobby. With my fresh water tank I was losing fish to flukes. I finally found a scientific article about H2O2 and flukes. To make a dip; 1tsp per gallon dip fish for 10 seconds and the flukes are gone and the fish are fine. I also give all plants be it a fresh water aquarium or pond a peroxide dip. I use a higher concentration and it prevents pests and bacteria from entering the system. I now also keep a small reef tank, and am trying to clear up the hair algae, and came across this article. I will definitely give it a try. 35% peroxide can be found in the laundry section. It is sold as a non-bleach. Oxi-clean is one but there are store brands as well. You have to be careful to read the label to make sure nothing else is added, It runs for about $3.00/liter in Canada.The above comments are for fresh water tanks as I haven’t tried it in my reef tank. I will say that I have used it on sand that was full of cyno. Living in a small town it’s not easy to get supplies, so I didn’t want to lose so much sand. I keep the salt water and sand siphoned out then dosed with peroxide. The next day the cyno was gone and the H2O2 has become H2O. I strained the sand and added it back to the tank and all was good. I wondered if the water would still be good but didn’t want to chance it. OK enough said by me. I have question what happens to the pods when you add a bit of H2O2 to the tank?


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