How to control hair algae in a reef tank

Hair algae is the common name for the green filamentous algae that grows in reef tanks. Its most common in new aquariums where the surfaces are new, clean and bright, the system is fully mature, and there is a lack of algae-eating livestock.

What causes hair algae in a reef tank?

Hair algae is caused by bright light and nutrients like phosphate, and it will often grow on rocks first but can grow across a sand bed if the sand is left undisturbed. The first thing to do is to control the amount of light that the tank receives. If there are no corals in there, have the light on just for viewing, and turn it off at all other times. If you do have corals in there, cut the total lighting duration down to eight hours per day – enough for corals but not an excessive amount that algae can take advantage of. 

Next look to control phosphates. Install a phosphate reactor and fill it with the appropriate amount of phosphate remover for your system volume. Hair algae need nutrients to grow and if you limit the nutrients you limit the growth. You can even fight fire with fire, introducing either an algae refugium or a macroalgae reactor in the sump. Macroalgae will compete with the hair algae for nutrients, helping to strip it from the water. It is also thought that macroalgae can release compounds that repel and retard other forms of algae in a natural form of chemical warfare. But once you’ve addressed light and nutrients, you’ll need some livestock to give you a helping hand 

Algae eating fish

Every natural coral reef has its army of algae eaters to keep algae at bay. Algae occur in the wild too, and when a reef overgrows with algae its often a sign of imbalance. If the tank is over four feet in length and 240 litres consider a yellow tang, which will graze rockwork for algae all day long, keeping green hair algae at bay as well as giving itself a feed at the same time. The best of the tangs is the Yellow eye or Kole tang, with specialised teeth to comb and clip hair algae like hair clippers. If tank space allows, a Yellow tang and a Kole tang can be used in combination to tackle algae, and they live alongside each other in the wild.

Rabbitfish like the Foxface are excellent algae eaters, if a bit large when fully grown, but will help to make short work of green hair algae.  


If you have hair algae growing everywhere, reach for a Sea hare. These large, brown sea slugs have enormous appetites for hair algae and will make a big difference in just a few days. They are so effective that many are returned to the store after a week or so as they have eaten all the available algae and have nothing more to eat!

To prevent hair algae growth in the first place, reef safe hermit crabs are the order of the day. They will scavenge the rocks all day long looking for all sorts of things to eat from uneaten fish food to various types of algae. Provide spare shells so that they don’t turn on each other or your snail population when they decide to move house. 

For stubborn algae consider a Sea urchin. Their tough teeth can even eat rock hard coraline algae, and they will take a thin layer of rockwork off as they graze your reef tank. Make sure your corals are glued down as they have a habit of bulldozing their way through them. 

What if nothing is eating it?

If nothing is eating it then it’s likely that it’s not actually an algae that your tank is suffering from, and instead, they are strings of inedible bacteria. In this case, make sure water parameters are optimal, phosphate is low as it possibly can be, and increase mechanical filtration and flow. If you can brush it off the rocks with an old toothbrush and syphon it off as you go, it will also help.

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Jeremy Gay is an author and freelance aquatic specialist. A former editor of Practical Fishkeeping magazine, he offers a wealth of experience on all things aquarium and pond.

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