Actually, the title of this article is a bit misleading. We can never "control" algae; we can only try to keep it "under control." To accomplish this, we must understand what conditions encourage the growth of algae and how they can be limited. The two factors that we must consider are light and nutrients.
Light
Light is one of the more perplexing components to algae control, as algae will thrive under low OR high intensities. Without aquatic plants, low light conditions will favor the growth of algae, since there is no competition for the light or other nutrients.
In freshwater planted aquariums, the use of full spectrum lighting will promote the growth of plants, which will restrict the growth of algae. If these bulbs are over 1 year old, loss of intensity might promote algae. If you notice this, replace the bulbs.
In most saltwater applications, the lighting intensity will be greater and if there are no competing organisms (corals, anemones), algae has all the light it needs. Many actinic and metal halide lights will require replacement after 6 months.
Nutrients
Almost all slime algae growth is caused by excessive nutrients, and true algae will also be more difficult to restrict if nutrient levels are too high. The two principal nutrients we need to control are nitrate and phosphate. Both of these are end-products of the fish and bacterial digestion of foods. Obviously, the less food we feed, the fewer nitrates and phosphate will accumulate in the aquarium. Since fish do need to eat, we need to take other approaches of control.
In freshwater aquariums, the presence of true aquatic plants will make better use of the nutrients, "starving" the algae. This is particularly true when we can keep the pH level between 6.5 and 7.0, where the plants will utilize the ammonium as a nitrogen source, but the excess ammonium will NOT be toxic to the fish.
In saltwater reef aquariums, the corals, anemones, and coralline algae will also out-compete the algae, as long as we keep the nutrient levels as low as possible. In the mini-reef, this means nitrate levels below 10 PPM and phosphate levels below 0.10PPM. (Even these levels are hundreds of times higher than the natural conditions. Paradoxically, the coral reef is actually a nutritional wasteland with an abundance of thriving organisms, thanks to the power of the sun.)
We can use phosphate removing pads or resins to help control the phosphate. The pads are used for a 72-hour period to reduce built-up levels, while the resins can be placed in the filter system for long-term control. To control nitrate, we must control the digestion of extra proteins in the water. In saltwater aquariums, we can utilize a protein skimmer to remove the proteins BEFORE they are digested. For most freshwater applications, this is not a practical solution. Another option is the use of protein-adsorbing resins which effectively prevent digestion of the proteins. As the resins become saturated, they will need to be replaced or recharged. Specialized De-Nitrators can also be used to "eat" the nitrate.
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