Posted by: joejaworski | 10-July-08

The Real Scoop On Brown Algae

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.


  1. Do these guys only occur in salt water? I’ve been setting up a 180 gal freshwater tank, or trying to, for the past 5-6 weeks. Having a lot of trouble with the filtration system (leaks and such) so no fish are in the tank yet; only rocks and a piece of driftwood and some aragonite. But that stuff in your picture looks very much like what i pulled out of the water a few weeks. ago. Ewwww! So gross.

  2. Brown algae in freshwater tanks are also diatoms. You might actually want to increase the lighting to promote good growth of the greens to compete for nutrients. I would also add an algae eater (pleco) since most plecostomas seem to prefer brown algae.

  3. […] is a quick discussion on diatom algae in newly established aquariums: The Real Scoop On Brown Algae Joe Jaworski’s Weblog The article seems to downplay the notion that silicate can leach out from the sand causing […]

  4. I have brown algae in my well.What should I do?

  5. Contact a full service well company that also installs water purification/filtering equipment.

  6. […] […]

  7. […] These are photosynthetic creatures and will not do well in subdued lighting" according to…n-brown-algae/ . Not only did I switch filters for a while, but I unintentionally left the lights off several […]

  8. First of all, I’m glad I found your blog. Pretty sure you have answered a question that has been nagging me for days. Where on God’s green Earth did my NO3 go?
    I am 28 days into a fish-less cycle of a 20 gallon fresh. I started at 4-5 ppm, add daily method. Once I got nitrites I dropped to half the daily amount of ammonia added. 14 days into the cycle, I got my first ZERO ammonia reading. Day 24 I had zero NO2. NO3 was over 160 ppm. Day 25 I had a massive diatom bloom. Day 26, EVERYTHING WAS ZERO!!! I have continued to add ammonia but still get 0,0,0. Everyone I have talked to has told me that my test was old and I need a new test, so I just jarred up some water and headed to my local pet center. His test said zero too. I have not done any water changes. I was planning to keep adding ammonia until I get fish, then do a 95% water change the day before I put them in.

    My questions are;
    Is it possible that the diatom bloom cleared my water of Nitrates?

    Should I wait for the green algae to take over before I do a water change and stop feeding ammonia? I’m planning to put a couple of Otto cats in to eat green algae anyway, so they will have a nice free food source.

    Do I even need to do a water change before I put in fish? Or can I just stop feeding the tank ammonia and put them in if my NO3 is still reading zero?

    Thank you for any help you can lend my experiment.

  9. Hi Joe, I notice your SEM photo of Diatoms. I am curious about the small solid spheroids in the low right corner and upper right corner. Do you know what they are? If you know, could you please send me a email? Thank you so much. These spheroids look so interesting.

  10. I know you wrote this blog post a long time ago, but I wanted to let you know that it really helped a lot! I’ve never heard of brown algae and I had no clue what was going on with my gravel. I blamed everyone in the house for over feeding my fish (thinking it was uneaten food), and then I blamed the filter itself ( a new filter that I assumed was spitting out muck). SO, the innocent filter and members of my household are now off the hook. 🙂 The only difference is the brown algae looks more like a sand color than dark brown, but I’m assuming it has to be the same thing because it’s all mucky and slimy, (you’re welcome for the mental image lol). Thank you for posting!!!!!

  11. […] This is because brown algae is actually a tiny little animal called a diatom.[1] […]

  12. […] This is because brown algae is actually a tiny little animal called a diatom.[1] […]

  13. I can also that the state with first-hand experience that the state of cycled tank makes a huge difference on how much, or if, brown algae forms. I have four tanks with the nearly identical water parameters and filtration systems. Only one of them is long-time cycled (9 months), whereas the others are only established 1.5 – 2.5 months (with ammonia and nitrites near zero). In all three of the newer tanks, I have had *lots* of brown algae for weeks even though nitrates are not excessive (30ppm or less). However, in the well-established tank (where nitrates levels are about the same — 20-30 ppm), there hasn’t been a trace of brown algae anywhere.

    Thanks for the article.

  14. […] nitrates and phosphates in the water are the main reason for brown algae on plants. Frequent water changes can reduce the nitrate and phosphate level of the water. This is […]

    • I’m assuming your talking about freshwater plants. This article and blog cover saltwater and reef tanks.

  15. […] This is because brown algae is actually a tiny little animal called a diatom.[1] […]

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