Every aquarium at one time or another experiences a bloom of brown algae. You are most likely to see it during the cycling phase of a new tank or while curing Live Rock. Brown algae can also show up at any time in well established tanks.

Brown algae is not an algae at all, but a tiny animal called a diatom. The animal is encased in a hard shell made out of silicone dioxide. The brown mats and film you see are composed of billions of diatoms interlocked together by their hard shells. They come in a variety of shapes and sizes and is the most common phytoplankton on the planet.

The appearance of brown algae in an aquarium is most often blamed on silicates in the tank water. Just like SPS corals that require calcium to grow, diatoms require silicates to grow. However, there are many other factors that cause brown algae and I believe the excess silicates story is way overplayed.

The use of “play sand” or silica gravel in a marine aquarium is often cited as a cause of diatoms. The silicates in these substances are bound up chemically much the same way as it is in glass. In fact, the glass panes that make up your aquarium is pretty much 100% silicates. The notion that play sand is a cause of diatoms is nothing more than another “reef legend”.

Almost every newly set up tank, during its cycling period, experiences a brown algae bloom. Even tanks with nothing but water and a layer of aragonite gravel will get it. Then if by magic, the brown algae begins to recede all by itself and is replaced by green algae.

During cycling, there is a time when the water contains high levels of dissolved organic carbons (DOCs) and nitrites, but low levels of nitrates and phosphates. It is these condition where diatoms seem to thrive. Minimizing their growth can be accomplished by turning the lights off during cycling. These are photosynthetic creatures and will not do well in subdued lighting. Secondly, performing large water changes and/or aggressive skimming during cycling will reduce their growth.

In established tanks, brown algae blooms are often times accompanied by some Cyanobacteria (red slime) as well. This is clearly an indication that it is an organics issue. Generous use of carbon and wet skimming will solve this problem.

When a brown algae bloom does occur, it is important to remove as much of it as possible. Similar to red slime, brown algae tends to “feed itself” through a die off and growth cycle that becomes self-sustaining.

Too much iodine in the water leads to brown algae. Even if you don’t use Lugol’s solution, check the label on any additives you use. Many contain significant amounts of iodine.

On rare occasions, excess silicates can be the cause. If you suspect this, phosphate removers also remove silicates. If you run phosphate removers all the time, it is highly unlikely that silicates are to blame. Still, silicates can get into your tank in a variety of ways. Using tap water or well water for topoff or salt mixing is a major source. Frozen Foods are another, though probably not a significant source.

Very high nitrate levels, even when phosphate and organics levels low will cause brown algae. No one knows the mechanism of this. Safe to say that It is prudent to keep nitrate levels low not only to prevent brown algae, but to prevent health problems with specimens in the tank.

Tanks with a good growth of green algae never seem to have brown algae problems. Perhaps the greens compete for nutrients, or more likely green algae consumes unknown compounds required for diatom growth. Whatever the cause, if one was to choose the lesser of two evils, green algae would be my choice.