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  1. #1

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    Default Algae: A How to Primer

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    Nothing sends more of a shudder down the spine of a fish-keeper than the word 'algae'. We've all sympathized with those whose tank looks like pea green soup. Those who have Cyanobacteria in such proliferation the fish are suffocating. Those who have foot-long hairs of black brush algae waving tauntingly from the outflow of a filter. Those that have the brown haze of diatoms covering everything that isn't moving.
    Keepers have torn their hair out trying to defeat the demon algae. In this post I'll tell you how to beat them all so you'll be able to save your sanity, and hairdo, and fall in love with your aquarium again.

    First, algae is a plant, a simple plant. We should give it more respect than we do, since algae filters the world. It cleans our sewage, out putting clean, clear water. It feeds hundreds of millions of Herbivores and Omnivores. Carnivores couldn't exist without getting their greens by eating those that eat algae.

    Algae filters aquariums remarkably well. My uncle kept a school of two dozen Neon Tetras in a 20 gallon tank for more than 12 years. The tank was bare-bottomed, and the only filtration was an ancient box filter. The floor and all the inside glass were completely covered with thick, green algae. You couldn't see the fish unless one looked down from the top.

    The water, however, was utterly and completely clear, so much so, the neons seemed to hang in space. By my readings, the water was so clean you could have dipped in a glass and drank it.

    And it was all due to the algae. My Uncle isn't a fish keeper. To this day he knows zero about what makes aquariums work. He just liked Neon Tetras when he saw them in a pet shop. He bought a tank, a cheap box filter and a stand so he could keep them.

    Fate favoring the foolish, my Uncle placed that tank opposite a west window and fed his fish. Naturally, it was soon covered with green algae on all solid surfaces.

    And his neons lived, and lived and lived. He donated them to a fish store when they were approaching their 14th year in his care. He was tired of not being able to see them through the glass. Gave me the tank and all his 'equipment'. Newly imported and starving loricariads had a ball in that tank.

    To my knowledge that's a record for Neon Tetra life span, and it's all due to my Uncle's naivete, and algae. Neons commonly live past 10 years properly kept and cared for, but I know when my Uncle got them and when he gave them away so they were at least 14.

    So when you see a little patch of green algae in a back corner of your tank, tip your hat to it. It's back there quietly metabolizing toxins and purifying your tank.


    Algae is a photosynthetic plant right? So if I turn off the light for a few days it'll go away right?

    Wrong. Algae won't go away and will return again and again until the root causes are addressed.

    And there can be several. Light plus high phosphate or plus high nitrate or overfeeding or plus old or wrong spectrum lighting can get algae going. And if the tank is exposed to even indirect sunlight, you'll be battling algae until the cows come home.

    It's simply a case of process of elimination. No sunlight on tank check, florescent tubes replaced last Tuesday check. Nitrate under 10 ppm, check. Culprit - high Phosphate.

    Natural Phosphate levels in fresh or marine water is .01 to .02 parts per million (ppm). In marine tanks, even .05 ppm Phosphate can spur a plague of algae, which is why so many small creatures are sold to reef keepers to help keep algae in check.

    Higher levels of Phosphate get into your tank water either by the metabolisms of the fish eating food or it's in your tap water, or both. Hobby level test kits don't have the resolution to measure Phosphate down to natural levels so though your kit may say no Phosphate, believe me, it's there. And it's very, very rarely down to natural levels in an aquarium.

    The solution is simple; filter the excess Phosphate out.

    There are many, many products sold to remove Phosphate. Nearly all are Iron Oxide pellets.

    One question I always ask of someone with an algae problem is if they use a canister filter (everyone should). If they did, they'd have a place they could place the pellets (bagged, of course), so they could reduce Phosphate to a point the algae couldn't get enough of it to live in such luxury. Established aquariums almost always have a bit of algae somewhere in their tank; those algae cells get just enough nutrients and light to survive.

    High Nitrate is another matter. Tanks that have high Nitrate are either not well cared for or has very large, messy eating fish.

    Regular partial water changes and regular vacuuming the substrate will do much to keep Nitrate in check. For those with large fish in large tanks, a Nitrate Reactor may be necessary to keep it at zero. An expensive solution, but one that always works.
    Weekly partial changes are best to dilute the Nitrate to manageable levels.

    Marine aquariums with live rock or reef aquariums with live rock and live sand rarely run into Nitrate problems as the organisms inside the sand and rock metabolize the Nitrate for it two Oxygen atoms, liberating pure Nitrogen gas. Other methods include deep sand beds and plenums (empty space under the sand) to eliminate Nitrate in tanks that have more numerous or larger fish in them.

    In lighting, the color temperature of sunlight, expressed in Kelvins, is 6700K, which is a bright, slightly yellowish light. It also is the color temperature that allows terrestrial and aquatic plants to photosynthesize the sugars the plants 'eat'. Conversely, aquatic algae prefers a blueish light, as the color blue transmits through water the furthest.

    Some recommend 10,000K lights for planted tanks. The light output is bright blue-white, like the mid-day sun in the tropics.
    Those that use 10K lights over their planted tanks almost always have algae problems. Remember, algae likes blue light.

    Algae shows up in well established tanks when the lights aren't replaced regularly. As the phosphors in florescent lights decay, the spectrum of the light shifts toward the blue. Those that change out their lights regularly rarely if ever see an algae outbreak over the life of the tank.


    Floating algae (green water) is almost always an effect of overfeeding your fish. Wrong light spectrum or exposure to sunlight can intensify the problem.

    The solution is simple; frequent partial water changes, a good gravel vacuum and a reevaluation of your feeding practices. All food should be consumed by your fish, no matter the size, in less than a minute. Some grazers, like catfish and herbivores, can be exempt from the one-minute rule.
    Promptly clean up ANY excess.

    If sunlight is also the problem, either move the tank or put up thick, dark curtains. If it is the old light that's intensifying the floating algae problem, change the bulbs.

    There's a quaint method of clearing a tank of green water using Daphnia pulex, a filter-feeding organism roughly the size of a flea. A troupe of these creatures will clear a tank of green water in days and prosper at the job, meaning you'll have many more Daphnia than you started with. A more modern method is using a flocculent product in your filter. The particles from the product bind with the algae cells, making them heavier than water, causing them to sink to the bottom of the tank.

    See Part 2
    When a finger points to the moon, the imbecile looks at the finger.

    Omnia mutantur nihil interit.

    The more you read, the more things you will know. The more that you learn, the more places you'll go

  2. #2

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    Default Part 2

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    But live Daphnia aren't easy to find in shops and the product to clear the water costs money. It's far simpler, not to mention basically free, to do the water changes, vacuum the substrate and not feed so much to cure the problem.

    As an aside, never, ever use algae destroying tablets. The dead algae will decay, and you'll have more problems than you started with if you do. It's also unnatural for a tank not to have some algae.


    Blue-green algae isn't algae at all; it's a photosynthetic bacteria. Cyanobacteria is also toxic to most fish and it also consumes Oxygen, meaning a full-blown invasion of Cyanobacteria can suffocate fish.
    It's also a slimy, smelly thing and many people find it disgusting.
    It almost always appears when the tank has either a short photo-period or low lighting.
    The solution is simple, but counter-intuitive.

    Increase your light. The more light you have the less likely you'll run into Cyanobacteria. If you lights are already bright, run them longer. If you have one or two 40-watt tubes, add another and run them 12 hours a day. Cyanobacteria is primitive enough it can't assimilate bright light.

    Bright light and manual removal will beat Cyanobacteria every time. Thankfully, Cyanobacteria doesn't bind tightly with solid surfaces so siphoning usually gets most of it.

    So if you run into Cyanobacteria in your tank, light it up and suck it up. May take some time to beat it totally, but it can be done.

    Interestingly enough, Spirulina, present in many fish and people foods, is also a Cyanobacteria.


    The pirate that is Black Beard or black brush algae isn't simply green algae run amok. It's a red algae of the Genus Audouinella and is totally a different kind of creature than the green algae we know.

    It almost always occurs either on the edges of live or plastic plants or in the flow of a power filter. The colors can be dark green, dark red or black.

    Removing it manually isn't the answer, because at the base of the hairs are spores, thus if you are careless in removing it, hundreds more colonies will appear.

    The answer is Carbon Dioxide. If you've pressurized Co2 in a planted tank, raise the level to 30 ppm. If unplanted or with plastic plants, take out the decorations and rinse them under a strongly running tap.
    If a planted tank without pressurized Co2 or one that the offending algae returns to even after the rinsing, Flourish Excel will kill the algae. Simply follow the directions on the bottle and start with daily half-doses, as full doses can damage the fish as well as the algae in un-planted tanks.

    You'll see the algae diminish nearly daily. When it's gone, it's usually gone for good.

    As an aside, tanks harboring Siamese Algae Eaters make such short work of any brush algae that appears it never gets a foothold in their tanks. SAE's (Crossocheilus siamensis) are the only fish I know of that eats it.


    Diatoms are usually the first algae to appear in a new freshwater or marine tank, and can cover literally everything that doesn't move. It's easily removed from the glass with a plastic scraper. Other surfaces can be more challenging to remove the offending diatoms.

    It shows up in new tanks usually within a month of setting them up. All it needs to get going is silicates in your water, a neutral or slightly alkaline pH, and the light that came with the tank.

    Thing is, over the next few weeks Diatoms go away by themselves. It uses up the silicates, without that to keep it going, Diatoms just fade away.
    Diatoms are only unsightly, not dangerous. Otocinclus will eat it. In marine tanks Diatoms are the first algae, followed by green algae and with live rock, coralline algae. Like freshwater, Diatoms fade away after a while so patience is the only weapon against it.

    These are the most common algaes you can run into during the life of your aquarium. With patience, all can be eliminated with minimal effort. So don't panic when algae rears it's head; you now know you can beat it.

    Last edited by Dave66; 06-14-2008 at 08:18 AM.
    When a finger points to the moon, the imbecile looks at the finger.

    Omnia mutantur nihil interit.

    The more you read, the more things you will know. The more that you learn, the more places you'll go

  3. #3

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    Kitchener, Ontario

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    Great article!
    I like algae, but then again I havnt had any big outbreaks. I like that it makes the tank look more natural. Sometimes I wish my gravel had more algae on it, it looks unnaturally clean lol
    30g - 4 female dalmatian Mollies, 1 female gold crescent Swordtail, 1 female pineapple Swordtail, 2 female black and white Platies, 3 female peppered Platy fry, 2 female Guppies, 1 male Guppy, 10 Harlequin Rasboras, 1 peppered Cory, 1 bronze Cory
    10g - 1 male Dwarf Gourami, 1 Synodontis catfish, 1 female shark-fin Platy, 1 male hi-fin Platy
    10g - 12 Red Cherry Shrimp, 36 Fry (Mollies, Platies, Guppies)

  4. #4

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    great article dave............
    angelcakes (penny)
    "The big fish eats the small one."
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  5. #5

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    Eden Prairie, MN

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    Thank you Dave66 for this article. This really answers a few questions for me. This is awesome.
    75g, 1 Flying Fox, 4 Siamese Algae Eaters, 4 Panda Cory's, 4 Peppered Cory's, 6 Danio's, 9 Hemigrammus Erythrozonus - Glow light Tetra's, 2 Albino Bristle nose Pleco's (Male & Female), 3 Upside Down Catfish, 2 Powder Blue Gourami's
    Live Plants with no Co2. Perfectly Healthy & Nicely Fertilized.

    Now with 1 baby Albino Bristle Nose.

  6. #6

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    I know this may sound silly, but i agree with luvfins, i like the way algae looks, in certain spots. So here is my naive question. Is there any way to grow algae, but keep it contained to certain areas? I love it on my gravel, and on the surfaces of some of the rocks in my tank, but obviously not on the glass. Is cleaning the glass my only alternative if i want this algae? or is there any way to contain it?

  7. #7

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    Virginia, USA

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    Thanks so much for all this real and practical advice on algae, Dave66! Very helpful!

  8. #8

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    Thanks dave this is most informative
    55 gallon Community
    4 dojo loaches
    3 sailfin mollies
    10 gallon Utility tank
    6 neon tetras to keep it cycled
    1 gallon Fry tank
    3 sailfin fry
    2 guppy fry

  9. #9

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    Jul 2011

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    Default Hello

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    Just joined the forum. I have a 30 gal tank with tetra 18. Water is cloudy after water exchanges, chem. normal. Could alge be a problem. Have use water clarifier without success. Thanks.

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