One of the main reasons that reef hobbyists give up their tanks is that they grow tired of constantly battling algae. Eliminating algae completely is quite difficult, but it is very possible to control its growth. This can be done once an understanding of what produces algae is achieved. The two major culprits for the algae problem are nitrate and phosphate. These are two of the main components in virtually all plant fertilizers and so eliminating or reducing them will reduce problem algae. In addition to promoting algal growth, these compounds have other properties that necessitate keeping their levels low.
Nitrate is the end product of the nitrogen cycle, which converts ammonia to nitrate. It is typically the form of nitrogen that accumulates in most tanks over time. It is found in virtually all protein-based foods, which is the usual source of introduction into tanks. In addition, local tap water is allowed to have over 40ppm of nitrate, so if unpurified water that is full of nitrate is used for make-up water, it becomes a problematic source as well. The desired level for nitrate in a marine tank is less than .5 ppm. Most test kits will be able to show even these minute amounts, and there are even advanced kits for hobbyists wishing to measure levels below these.
Nitrate is not only a stimulant for algae, but has also been shown to be a stimulant for dinoflagellates and to have other negative effects on a reef tank as well. When nitrate levels are high they can limit the growth of stony corals by causing an increase in zooanxthellae (a dinoflagellate). This produces two effects: it causes the stony corals to be browner and it slows or reduces calcification. These effects can be seen when nitrate levels are even slightly higher than .5 ppm. Fish seem to tolerate nitrate much better and have been raised from larvae successfully even when 40ppm of nitrate was present. However, there have been anecdotal reports that when nitrate levels were high the coloration of the fish diminished and in some instances the fish went blind.
For these reasons, it is a good idea to keep nitrate levels as low as possible. One way to do this is to reduce food input. Overfeeding is still the most common cause of excess nitrate. The second common cause is high nitrate levels in tap water, so a reverse osmosis deionization unit should be used with any water where nitrate is a problem. Improved skimming can also help to reduce excess nitrate. It will not eliminate all excess nitrate, but can reduce it significantly. It is also possible to remove nitrate by adding compounds such as AZ-NO3. This additive contains some compounds that bind to nitrate and make it easier to be skimmed off with a protein skimmer. There are also resins available that will reduce nitrate. However, it should be understood that these resins are short-term fixes and are not cures for the problem. Nitrate-producing filters such as trickle filters should be replaced with live rock and protein skimmers if nitrate becomes a problem. Lastly, a Caulerpa-based refugium, a deep sand bed, or a denitrifying filter can be employed if the other means do not reduce the nitrate levels. Each of these has positive and negative attributes which should be noted and taken into account before they are deployed.
Like nitrate, high phosphate level stimulates excessive algal growth. In addition, high phosphate can significantly inhibit calcification by stony and soft corals, coralline algae, and other calcareous organisms. Unfortunately, phosphate is much more difficult to measure for accurately than nitrate is. This is partly because phosphate is present in two forms in reef aquaria: inorganic orthophosphate and organic. Most test kits only measure inorganic levels and not organic levels, and in most instances these inorganic levels are usually very low. This may be due in part to the fact that algae consume orthophosphate as quickly as it is available. As a result test kits read little to no phosphate, but algae is a continuous problem. Similarly corals may look fine, but their growth will be slow when phosphate levels are above .005, which are difficult to measure with most test kits.
Fortunately there are ways to reduce phosphate, many of which are similar to those used to reduce nitrate. Like nitrate, organic phosphate can be skimmed off with a protein skimmer. Organic phosphate is removed as part of the dissolved organic compound complex targeted by efficient protein skimming. Similarly, phosphate is taken up by macroalgae, so a refugium housing macroalgae can be employed to help with phosphate export. Tap water can introduce phosphate so water purification should be employed. The use of kalkwasser has also been found to cause phosphate to precipitate out along with calcium carbonate. Lastly, there are now very efficient resins for removing phosphate. These used to be aluminum oxide-based and by design had to be removed on a regular basis after just a short period of time. This made their long-term use impractical. Recently a new generation of iron oxide-based phosphate-removing resins has come on the market and these are useable on a continual basis. Some of these compounds appear to be capable of lowering phosphate levels by a much greater degree. The efficacy of these resins should be researched before they are used.
Controlling nitrate and phosphate to stem or eliminate algae growth is now possible. Understanding what causes these compounds to be present is the first step to eliminating this problem. Fortunately, due to improvements in this understanding and the advent of new products, it is now relatively easy to get algae growth under control.
Mike Paletta is the author of The New Marine Aquarium and Ultimate Marine Aquariums. He has been in the hobby for over 15 years and has written numerous articles for Aquarium Fish Magazine, Tropical Fish Hobbyist and Aquarium Frontiers.