Every saltwater aquarist experiences green hair algae in their marine aquarium at one time or another. It may start as just a few strands of algae on a piece of live rock and seem to not be a big problem, but if the conditions in the aquarium are right, it can spread across that one piece of rock, then leap to other pieces, and then quickly jump to the aquarium walls and cover them.
Most people will try to scrape it off the glass with algae cleaning magnets, brushes, or even razor blades, but that only slows it down or can actually make it spread even faster as it disperses bits of algae to the rest of the tank.
Taking the rocks out of the aquarium and scrubbing them seems to help for a bit, but it will usually come back in a short period of time unless the rock is literally sterilized with an oxidant like bleach. Even then, if all of the algae is not removed from the tank, it will start all over again.
Slow Down the Algae Growth
The good news is that there are a number of things that you can do to your tank which will slow down the algae growth.
Green hair algae requires light to grow, but greatly reducing the light in a reef tank can have a detrimental effect on the corals in your tank. Some light reduction will help reduce some growth, but not significantly.
Reducing the foods that the green hair algae requires to grow will stunt its growth. Most algae feed on nitrates and phosphates so removing these from the aquarium water can make a difference in its growth rate.
But, it is virtually impossible to remove all nitrates and phosphates from your tank water. Everything that you put into your tank, including fish and critter foods, will raise both the nitrate and phosphate in the tank water. Both natural sea water and the sea salt mix that you buy to make the saltwater you put in your tank contain nitrates and phosphates.
A good place to start is getting a handle on decomposing food and detritus. Both generate nitrates and phosphates, so keeping your substrate clean is imperative.
Clean Substrate and Reduce Algae Food Source
How do you keep your substrate clean in a saltwater aquarium? You can siphon clean it, but that can be a pain as well as difficult to do in a reef tank since all that live rock gets in the way. Fortunately, nature has provided critters that will do the job for you. This is not only efficient, but fun to watch as well. A downside of using these living cleaners is that they require smaller grain sand (not crushed coral) in order to pass the sand through their gills. The sand sifting sea star does a great job of gobbling up stuff from the substrate, however, it will also consume all of the live material in live sand at the same time. There are a number of reef safe hermit crabs that do a great job of cleaning the substrate, also.
There are a number of marine fish that consume green hair algae. Some of the reef safe algae eaters, such as the lawnmower blenny (Salarias fasciatus), yellow tang (Zebrasoma flavescens), blonde naso tang (Naso lituratus) and a number of others do a great job. Sand sifting gobies are fantastic sand scrubbers. They gobble up the substrate, suck whatever non-sand items are in it, then spit the sand back out through their gills.
You can perform endless water changes to reduce your phosphate and nitrate levels, but many have found that the vodka method for nitrate and phosphate reduction works well if you have a protein skimmer in your system. There are a number of nitrate reducing products which also reduce these compounds.
Green Hair Algae Imposters
There are quite a number of different species of algae that look very much like the typical green hair algae found in saltwater aquariums but are not the same. The differences can only be identified via a microscopic examination by someone who knows what they are looking for. These different types of algae, even though they look like green hair algae, are distinctly different. Algae eating fish and sea creatures will not consume this type of algae. If you are experiencing this algae and no eradication method seems to work, your only remaining course of action is to strip the tank and scrub everything down.