How to Get Rid of "Brown Algae" (Diatoms) in Freshwater Aquaria

"Brown algae" on tank ornament
"Brown algae" on tank ornament | Source

Brown algae can be a delicious meal or an unsightly oxygen hog, depending on the type of animal you choose to keep in your aquarium. When aquarium owners talk about "brown algae," they are actually talking about diatoms, microscopic brownish or greenish creatures with glassy skeletons. (When botanists talk about "brown algae," they are likely talking about distantly related large seaweeds.)

Understanding "Brown Algae" (Diatoms)

What do "brown algae" need to live? It's best to always remember that although diatoms can and will synthesize food using sunlight (or almost any light, for that matter), they don't need light the same way regular algae or plants need light. That is to say, the diatoms don't get energy exclusively from light itself.

Here are conditions that are beneficial to diatoms:

  • Light
  • Silicates (rocks or sand containing silica)
  • Nitrates (a waste product of aquarium life)
  • Still or stagnant water
  • Hard or mineral-rich water

The two main differences between true algae and diatoms are that regular algae do not consume silicates and that regular algae can be eradicated by eliminating either the light or the nitrates. The problem most people run into with diatoms is that no matter how long you leave the lights off, the diatoms persist. And if you were able to somehow eliminate most of the nitrates from the aquarium, the diatoms would consume silicates as well, or instead.

So the first step to getting rid of brown algae is to erase the word "algae" from your thinking about this stuff.

Step two is to make sure the tank has adequate filtration (the filter system is rated for the gallon capacity of the aquarium). The aquarist should keep in mind that many of the "hang on the back" style filters can expose the water to a very large amount of light, depending on the brand of filter and the ambient lighting in the room.

Step three is to realize that even though you may have adequate filtration, you need water movement as well to discourage diatoms. You can get water movement using products called "powerheads" or "current makers." These move the water around in the tank, accomplishing two things:

  • Diatoms aren't very strong and they don't swim. A current maker will prevent them from anchoring anywhere, thus preventing their growth.
  • The added water movement will keep all the water in the tank filtered, instead of just a percentage.

For step four you need to do some hands-on work with the tank. Very carefully remove all plants and ornaments from the tank, leaving only substrate (rocks or sand) and hardware (filters, heaters, and so on). Wipe the diatoms off the surfaces of the hardware and try to scrub it out of the rocks or sand to the best of your abilities. You could bleach all the plants and ornaments at this time, or you could scrub them with an abrasive sponge. Either way, get the diatoms off the plants and ornaments while they are NOT inside the tank.

Step five should be pretty obvious once you remove everything from the tank. Do a 30%-50% water change (after the hardware and substrate have been scrubbed). Then return all ornaments and plants to the aquarium.

Step six is the last step, and the same as the last step on a shampoo bottle. REPEAT. Do this about every two weeks and you'll see the diatoms bloom less each time (you should only have to do it two or three times).

If these steps don't work for your aquarium, it may be that:

  • your aquarium is exposed to too much light,
  • there's not enough water movement in the tank,
  • you keep scrubbing the diatoms off into the tank water, or
  • there are too many diatoms in the substrate.

"Brown Algae" in a Saltwater Tank

Cleaning a Freshwater Tank to Remove "Brown Algae"

Comments 7 comments

David 3 years ago

That doesn't explain why they even grow on my air stone? Shouldn't there be enough water movement around a powerful enough air stone?

vic 2 years ago

Rather odd but mine grows only on water heater that sits in front of return from canister filter

Derek 24 months ago

This is horrible information. If you are having a problem with brown algae, please seek other information. The techniques listed here will do very little to control or eliminate diatoms.

Katie 22 months ago

Very helpful...We have very alkaline, mineral rich water (brought in fish from the pond); I have not been able to get rid of this "stuff" but I have just been scraping it off and hoping the filter and a few snails would take care of it. So, does anything eat diatoms? The snails have eaten but I'd need about 50 good sized snails in a 30 gallon to make a dent, at the rate the two I have are going at it; of course, that is ridiculous! I have very mixed feelings about bringing these beautiful, fancy "gold fish" home...

Anonymous Coward 7 months ago

@Katie Goldfish are notorious for the mess they make. In order to keep a sightly tank populated with goldfish, you need an oversizeed water filtration system. Plus, you should regularly scrub the substrate (there are machines like sand vacuum cleaners for aquariums - essentially, a pump soaking up water from the bottom of the tank through a sieve fine enough to let only silt and dirt pass through but not sand, then filtering it through some finer filtering material than that in your tank filter - often some synthetic fibers that make a material like cotton wool but don't stick together when wet). With goldfish, you also need to heavily secure plants at the bottom of the aquarium - they dig out everything that's not fastened.

But they are gorgeous fish ...

Richard 7 months ago

Can not say I agree with the original author. I have hard water and just cleared a new set up OF DIATOMS that has been going FIVE months. I found Ottos' and black mollies helped a lot but I found ( 4 ME) that less food and repeated spot treatment ( aggressively for 2 weeks daily) with Hydrogen peroxide did the trick. You have to turnoff filters and all water moment. Do not exceed 3 ml per gallon ever. Typically i a daily treatment on 555 I used maybe 30-40 ml tops and worked my way across the tank.

Rev. Leonidas 5 months ago

Moss balls attract diatoms; rinse these daily with some of the aquarium water that is being removed daily.

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