For many hobbyists, unwanted algal growth in aquariums can be a real nuisance. But, keeping it under control isn't so difficult if you know what to do. Whether it is fine microalgae growing all over the glass panes, on rocks and decorations, or on top of the gravel/sand substrate, using the right tools, techniques, and helpers can make things far less frustrating, and keep your aquarium looking its best. So, let's go over some of the things you need to know to keep it at bay.
Regardless of what type(s) of alga you find growing in your aquarium, keep in mind it isn't so different from "regular" plants when it comes to basic requirements. They all need light and they all need nutrients. In addition, there is probably something that likes to eat them, too. Thus, we need to look at each of these and see how they apply to control.
Lighting first. If you have a non-reef/fish-only aquarium with no light-dependent livestock, you can simply leave the lights off as much as possible. Like I said, algae need light, so if it is showing up, just try keeping the lights on for a shorter period every day. I usually advise hobbyists to leave the lights on from the time they get home from school or work until they go to bed - but not all day while they aren't at there. Also, if your aquarium receives a great deal of sunlight where it is located and you think this is spurring growth, you can get some good window blinds/curtains, or move the tank to a darker area. The fishes will get some ambient light in most any room, and they don’t seem to mind being in the shade for most of the day. Depending on how long the lights do stay on and how much other light the tank gets, this can make a huge difference in how your aquarium looks without having to do anything else at all.
Conversely, if you own a reef aquarium, you’re obviously stuck with having very intense lights that have to burn for several hours a day to keep the things alive that you want to keep alive. There's not much you can do about that without interfering with the health of any sorts of symbiotic corals and such in the aquarium. Thus, you'll obviously have to rely on other means of control.
Of course, even if you can keep lighting to a minimum, that isn't always enough. You'll also need to maintain good water quality to help, as the nutrients that algae need are found dissolved in the water itself. When it comes to these nutrients, it is phosphorus in various forms that is the prime fuel for growth. Phosphorous makes up part of any sort of fish or invertebrate food that you add to an aquarium, and it also makes up a large portion of most any plant fertilizer. So, as you can imagine, it is essential to keep concentrations at a minimum.
To start, it is very important that you make sure not to over-feed the animals in your aquarium. Common advice is to refrain from adding more food than your fishes can eat in one to three minutes, but I tend to look at the fishes and not my watch, and feed them until they seem happy. Note that I do this by giving a little food, and a little more, and a little more until they are okay, rather than dumping in one big load at once. How much and how long you feed will be up to you and your own judgment, of course, but the important thing is to make sure that they don’t leave any leftovers. The same applies for any sort of invertebrate foods, etc. that reef aquarists may be using. Any leftover food that doesn’t get eaten by something somewhere in the aquarium will break down and increase the concentration of phosphorous in the water. Do keep your senses about it though and realize that starving your fishes to death to quell an algae problem is not the way to go about it.
With that said, the fish will nevertheless eventually release all of the food you did give them in a different form. We tend to think if it as waste, but it still has phosphorus in it, too. Once fish waste is in the water or on the bottom we call it detritus, and keeping detritus from accumulating is the next goal. For everyone that uses some sort of mechanical filtration, like a sponge or floss filter, it is imperative that you clean it out regularly. Pull the filtering media out as often as possible and rinse it out thoroughly. This will keep detritus from building up in the media and thus keep it from slowly releasing phosphorus back into the aquarium. Letting such a filter run for long periods may help keep the water clear, but it isn’t going to do anything to help keep algae away.
Using a good skimmer (foam fractionator) can help to maintain excellent water quality and help to keep phosphorus concentrations down, as well. The nasty sludge that builds up in a skimmer's collection cup is comprised of all sorts of junk, and is also rich in phosphorus. So again, the nutrient is being removed from the aquarium instead of building up in it.
Next, using a specialized phosphorus-removing product can also help. These products are appropriately called "phosphate removers" and they’ll extract some phosphorous-based compounds from the water. When enough is used they can have such a dramatic effect on phosphorus concentrations that I’ve used them in aquariums that literally had green water and seen them clear up completely in a couple of days. But, such products shouldn’t be thought of as a "magic bullet" that will fix everything and allow you to dump phosphorus in your tank at will. They only remove certain types of phosphorus-based compounds, not all of them, and they tend to be rather costly, as well. In addition, many hobbyists have reported that the use of phosphate removers that contain aluminum can have strongly adverse effects on a variety of soft corals. This has recently been tested and verified experimentally*, so if you choose to use such a product and you see problems with soft corals, remove the stuff immediately and find a brand that is aluminum-free.
Last, but certainly not least, you can (and definitely should) perform periodic partial water changes, using purified water. I strongly recommend doing water changes to improve water quality, using phosphorous-free water produced through reverse osmosis filtration or distillation. Each time you use purified water to make up synthetic seawater and add it to an aquarium, you'll be diluting whatever phosphorous there is and thus lowering the overall concentration. The more water you change, the more the concentration goes down.
Now, with these tools and techniques covered, we can look at some living organisms that can help out. For starters, you can use other types of algae as competitors and as a means of phosphorus removal. Secondly, a large number of sea creatures make their living by eating algae, and they’ll be quite pleased to do the same in your aquarium.
You may be able to keep a variety of attractive macroalgae, such as Caulerpa, growing in your aquarium. The macroalgae will use the available phosphorous like any other algae, and will incorporate it into its own living tissues. Thus, as is it grows, more and more phosphorous will be taken up. As it continues to grow, you can then pluck out clumps and strands here and there to keep it under control and in doing so will be physically removing nutrients from the system. The macroalgae suck up the phosphorous, you pull out some of the macroalgae. It’s that easy. The only problem is that you’ll need sufficient lighting (as in reef-type lighting) to keep it growing well.
If you don’t like the idea of having a bunch of macroalgae growing in your beautiful reef tank, you can always grow it in a second tank hooked up to the main one. Such tanks are often called refugiums or an algal scrubbers, and they can hold the macroalgae out of sight under the main tank, or can be used as an additional display tank for other types of critters. Many people have chosen to use algal scrubbers as a primary means of aquarium filtration, so if this sounds like the way to go for you, I strongly suggest you do some further homework on the topic.
As far as using algae-eating animals goes, herbivorous fishes can be invaluable. Surgeonfishes (tangs) are a popular choice, as they will nibble around the tank all day. Most will pick at microalgae on the glass and the rocks/decorations day and night. Personally, I absolutely love algae-eating blennies, too, and don’t think red-lipped blennies (Ophioblennius atlanticus) and lawnmower blennies (Salarias fasciatus) can be beat. They have big mouths and stay busy, and like many other blennies, both have unique personalities. Even though they aren't the most beautiful fish around, you'll enjoy watching them and they’ll help earn their keep at the same time. Really, there are entirely too many algae-eating fishes to list off, but tangs and these blennies are indeed some of the best.
There are also quite a few crabs that will go after unwanted algae. Emerald crabs (Mithrax/Mithraculus spp.) are great at picking filamentous/hair algae, and some will even eat bubble algae (Valonia spp.). The only problem you may have with these is that some get big enough to knock stuff over. And, while I’ve never seen it myself, I’ve heard that big ones can grab fishes and make an occasional switch to being carnivorous. So, you may have a small one for a while and then find the need to pull it out later.
The same goes for the Sally Lightfoot/spray crab (Percnon gibbesi). They also make great algae eaters when small, but may get big enough to try to eat your fishes. I once read about one that liked to climb up onto rock perches and jump out at fishes when they swam close enough to be grabbed! However, I think this is exceptionally unusual, as I've never seen one bother anything at all (except algae).
Small hermit crabs can be great, too. The blue-legged (Clibanarius tricolor) and red-legged/scarlet hermits (Paguristes cadenati and Calcinus spp.) are at the top of the list, all of which will safely pick away at various sorts of algae found growing on the surface of live rock, gravel, and other decorations. These guys are also small enough in size that they won't knock anything over, or bother any of the other invertebrates either.
If you’ve got plenty of algae growing on the rocks you can always try a herbivorous sea urchin, too. Several common varieties are great cleaners which strip away tracts of algae, but note that they can pose a couple of problems though. First, they are typically covered with long, sharp spines, which can spear anything they happen to come across, including your hand. I haven’t had any problems with them sticking any other critters, but I have had the misfortune of accidentally bumping an urchin on more than one occasion. Quite painful, indeed. And, they also have a specialized grinding apparatus for collecting food that works so well that they can grind patches of your live rock or decorations into sand. Any desirable, colorful coralline algae present may be stripped right away during the process. In a big tank with plenty of rock this may not even be noticeable, but in smaller tanks it can be irritating to see your rock, etc. slowly being chewed away.
Lastly, we get to the ever-popular algae eating snails. Turbo snails (Turbo spp.), astraea snails (Astraea spp.), and trochus snails (Trochus spp.) are the most common varieties, which will crawl all over and clean up every surface in a tank. If you should catch one moving across the glass you can even see its strange little mouth moving in a side-to-side pattern, slurping it clean and leaving a signature feeding trail behind. One thing to mention though is that of the three I have found that the larger turbo snails are not necessarily the most desirable of the bunch because they can get big enough that they may knock things around pretty badly at times. Astraea and trochus snails on the other hand, stay much smaller and are thus less likely to cause any problems.
That’s about it though. So, now you need to think about how you can use any or all of these different tools, techniques, and creatures to fight any algal problems you may have. Just remember that no matter what you do, unfortunately, you’ll probably still have at least some unwanted algae showing up here and there. But, that’s just a part of having an aquarium.
*Holmes-Farley, Randy, 2003. Aluminum in the Reef Aquarium. Advanced Aquarium Magazine: http://www.advancedaquarist.com/issues/july2003/chem.htm