The most common nuisance algae we experience in the seahorse tank are Cyanobacteria (red-slime algae) and Derbesia sp. (green-hair algae).  Both are ectophytic (living and growing on the surface of other organisms), and this is generally why they are problematic.  Cyanobacteria, typically is associated with overfeeding and elevated nitrites (NO2), forms mat-like coverings on the rocks and substrate, and are often found growing on gorgonians, sponges, and even seahorses! The primary concern of Cyanobacteria in the tank is the suffocating effect is has on the organisms it covers, as it impedes natural metabolic processes of more beneficial microbes and other algae.  Cyanobacteria can be easily taken care with water changes and physical removal of the mats and gently suctioning it off of other organisms.

That leads us to the dreaded green hair algae, Derbesia sp.  It should be noted that there is another relatively common green hair algae called Bryopsis plumosa that is much less aggressive, and far more attractive than Derbesia sp., Bryopsis plumosa is a feathery-leaved algae, very delicate in appearance, and generally more bush-like, whereas Derbesia sp. is a long hair-like algae with no discernable "leaves".  From this point on, I will use the term "green hair algae" to refer to Derbesia sp. for readability.  Green hair algae can quickly overrun a seahorse tank, covering rocks to the point of completely obscuring them from view, choking soft corals and corallimorphs out of valuable real-estate, and suffocating sponges and gorgonians to death.  This is probably the pest you are dealing with if you have read this far, but, please bear in mind that even though this algae can be a burden and a potential killer, it is in fact beneficial in that it removes nutrients from the water such as phosphates and nitrates.  It also takes CO₂ from the water and returns O₂, just as any plant will.  However, in the scope of this article, we are discussing algae levels that have grown beyond the aesthetic value and are impinging on other, more valued organisms.  Essentially, we are treating it as a nuisance.

So, that said, how do we deal with this stuff?  It grows, practically right before your eyes, and nothing seems to help.  When you remove it is comes right back, sometimes even worse!!  Ack!  Ok gang, here is the scoop.  This is based on my own trial and error, research, and experience.  First, a green hair algae outbreak is generally a function of three conditions plus a catalyst.  What this means is that a tank can have the following three conditions and never experience a hair algae outbreak, but if the process is catalyzed then you get problems.  Inversely, if you have the catalyst and lack one or more of the conditions, chances are you will not experience problems either.

Let us first examine the conditions so that we have a more thorough understanding of what we are up against.  This will better equip us to deal with this problem in very specific ways.

The Catalyst
The catalysts in this case are green hair algae spores called "sporangia", which are like algae-eggs waiting to hatch.  When they mature and burst open, they spread the algae throughout the tank.  Unfortunately, most aquaria already have these spores in them, either in the water column or on the rockwork.  Granted, there are cases where these spores would not be present, say with the use of a UV sterilizer and micro-filtered seawater or synthetic seawater.

Condition 1 - Phosphates
Phosphate (PO₄) is the primary food source for green hair algae.  Unfortunately, many of the foods we feed our seahorses in order to maintain good nutrition decompose and leave behind large quantities of organic phosphates.  So, while we are busy plumping up our horses, and making sure they are getting proper nutrition, we are, consequently, creating a phosphate sink in our aquarium.  Don't be surprised if your tank has a green hair algae problem and your phosphate levels test out somewhere around 3 or 4ppm.  Green hair algae can grow and thrive in the presence of <1ppm!  So, we need to endeavor to keep phosphate levels below 1ppm, or better still, below 0.5ppm.

One of most common ways for phosphates to build up in the seahorse tank is from overfeeding.  Since seahorses are slow eaters, often food will be whisked away by the currents before they have a chance to get to it.  Without an adequate clean-up crew, this food will decay in the nooks and crannies of the rocks and substrate, leaving behind little phosphate deposits that will leach out into the water.

Condition 2 - Lighting
Algae of all types need light in some degree.  Green hair algae thrive on more-red spectrum lighting, which is why it is so often a problem when our lights get old, and red shift.  As lights age, they tend to shift toward the red-end of the spectrum.  This means that the older the bulbs get, the more red-spectrum light they are emitting, which is hard to notice because it is such a gradual change, but the algae knows! Often times, algal overrun coincides with the winter of a light bulb's life.

Condition 3 - Molybdenum Green hair algae use Molybdenum (MoO₄) as a food source.  Molybdenum (pronounced muh-LIB-de-num) commonly enters your tank in trace element additions and water changes.  Molybdenum is essential to beneficial organisms such as zooxanthellae, as well as nuisance algae such as green hair and Cyanobacteria.  However, molybdenum in concert with high phosphate levels and ample lighting can cause an explosion in the density of green hair algae.

How to Do Battle Here is a list of things you can do to help overcome the algae.  The more of these you can do, consistently, the quicker the algae will subside.  Any of these techniques will help, but the concert of all of them is designed to eliminate what the algae needs to grow, and thus suffocates it for vital nutrients and critical light for photosynthesis.

• First and foremost, do not overfeed, and have an adequate clean-up crew.  For the first few days of feeding your horses, watch them closely (I know this is a lot to ask… sit in front of the tank watching your spectacular seahorses in rapt amazement, but you are just going to have to do it!), and see how much they eat.  Feed only a few pieces of food at a time until you see how many pieces each horse will eat.  This will give you an idea of how much to feed for your whole herd.  Target feeding can also help in that you know right where the food is going.  As an additional aid, you might consider turning off the filtration for a half-hour during feeding time to allow the food to settle in place and not get blown into impossible-to-reach areas of the tank.

  Clean-up organisms in the seahorse tank are crucial, due to the limited water flow and types of food the horses need in order to remain healthy.  Small hermit crabs (scarlet-reef hermits, blue-legged hermits, and left-handed hermits are among my personal favorites), as well as Nassarius sp. snails can make for an excellent group.  One per two gallons or so is my personal rule of thumb for stocking levels on these guys.  Other algivorous snails like Trochus sp. and Astrea sp. are excellent additions to help maintain micro-algae that will eventually release phosphates back into the water.  Herbivorous fish such as Tangs, Rabbitfish, and the like are not recommended for the seahorse tank as they can get quite aggressive and will easily outcompete the seahorses for food.

• Replace your light-bulbs.  Remember, lights red-shift as they age, and red-spectrum light is essential to algal growth, so if your bulbs are a year old or more, replace them.

• Get some PhosGuard (or other phosphate absorber).  I recommend this brand over all others including the phosphate pads, as I have tried the vast majority of those commonly found on the market, and PhosGuard reigns supreme! You can get it just about any aquarium store, however, if you go to That Pet Place (www.thatpetplace.com, 800-786-3829), you can get a 4-liter tub for $29.99, which is very cheap (it normally runs around $39.99).  Four liters may seem like a lot, but you will be changing this stuff every week until you get the phosphate levels under control, and then you will want some on hand for maintenance.  The 4-liter size will treat and maintain a 40-gallon tank for approximately two to three months.

  While phosphate removers will pull out a LOT of phosphate, they do not work overnight, so be patient.  In a tank with an algae problem it may take a couple of weeks for the levels to come down.  You can speed this process up with weekly water changes.

• If you can, do not run your lights at all.  This means, if you have no corals, macro-algae or other organisms that will suffer from this.  Alternatively, you can run only Actinic bulbs.  If you don't already have them, you might consider replacing your lights with pure Actinic bulbs.  The blue-spectrum light will provide the necessary light to your desirable organisms and reduce/eliminate red-spectrum light used by the nuisance algae.

  As a compromise, you can reduce your photo-period by 2/3 or so, however, even though this may benefit your photosynthetic organisms, it will also provide much needed light to the nuisance algae, and thus make the battle that much tougher to win.  If you do decide to reduce of eliminate the photo-period, you may wish to increase the feeding of your corals.  Be careful not to overfeed them or this will perpetuate the existence of the nuisance algae also.  If you normally feed your corals twice a month, you may feed them a third or fourth time while the lights are out.  This will help sustain them through the darkness.

• Stop delivering molybdenum to the system.  Since most trace element additives have molybdenum in them (even those Sea Lab #28 blocks that a lot of seahorse keepers use), this will be difficult to do without the temporary cessation of trace element addition.  Your more desirable organisms may suffer slightly, but no more so than they are while being overrun and suffocated by the algae.  Once you get the algae problem under control, then you can return to adding trace elements.

• Hand prune the algae.  Either pluck it or use a toothbrush to scrape it and remove it.  This is critical.  Algae, when killed, releases PO₄ into the system, so you can create a self-perpetuating cycle if you just scrape the stuff off and leave it floating in the tank.  I like to do this just before doing a water change, so that after removing as much of the algae as I can by hand, I can get most of the particulate algae in the water column out with the water change.

  In addition to pruning the algae, you can also take a bucket of water that you have siphoned out of the tank during your water change and remove rocks to it, scrubbing each on with a toothbrush before returning it to the display tank.  This will help remove additional algae strands as well as those tiny sporangia, and since you are doing it in the bucket, the removal is greatly enhanced.

• Finally, encourage the growth of coralline algae by maintaining proper levels of calcium and alkalinity.  Although green hair algae are ectophytic, it will generally not grow on coralline algae for some reason.  However, it should be noted that green hair algae sporangia (the spores that spread the algae) can be found on coralline algae.  So, the more surface area covered by coralline algae, the less surface area available to the nuisance algae to take hold on.

I wish you all luck in battle, and the happy return of your beautiful seahorse tanks!

References

Goemans, Bob.  2000  Marine Algae Control Secrets - Remedies for the Marine and Reef Aquarium
Sprung, Julian.  1989  Reef Notes (on green hair algae)