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Managing Blue-Green Algae in a Freshwater Aquarium: Keeping Unsightly Algae Out of the Fish Tank

Updated on May 12, 2011

Microscopic cyanobacteria


Aquariums can be beautiful additions to your d├ęcor and learning how to properly care for the fish can be a very fulfilling and enjoyable task. However, as with any pets fish require routine care including regular water changes. Just like any cat box waste accumulates fast, if your tank is properly cycled you may not actually see any signs of a dirty tank but don't be fooled...a perfectly crystal clear tank can still have lethal levels of nitrates in the water. One sign that your tank may not be maintained properly is an excess of algae, most notably either a brown algae or a slimy type called blue-green algae (though it can be dark green or black). Blue-green algae can be unsightly in your tank and can also be hazardous to the fish inside. Let's begin with a couple of things that do NOT work to get rid of blue-green algae:

Algae-eating fish - Common algae eaters such as plecostomus, otocinclus, and grazers such as goldfish or gouramis will not touch the stuff. Why? It isn't really algae. Blue-green algae is a cyanobacteria that is exceptionally difficult to get rid of once it takes hold but definitely doesn't taste like the "real algaes", or actual plant algae. I have noticed that a lot of my apple snails enjoy munching on the blue-green algae but never do well with completely getting rid of it.

Commercial algae killers - Alright, so there are some times that these products will work to get rid of the algae temporarily because they are very hard on small animals...and thus hard on blue-green algae. Unfortunately they're also very hard on the beneficial bacteria that breaks down the ammonia from the fish's waste into less toxic nitrites and then into even less toxic nitrates. Additionally, many of those products contain copper which will kill invertebrates such as snails and shrimp and isn't very healthy for the fish either. I say "temporarily" get rid of the algae because it won't do any good to kill existing algae without looking at the root of the problem.

So what do we do about it? While it's difficult to get rid of the algae altogether close attention to the tank's maintenance can limit the algae and keep it from reaching harmful levels in your system.

Partial water changes should be done about every week, more or less depending on your tank size and stocking level. Many aquarium books and sources will tell you to keep your nitrate levels at 40ppm or less, I raise snails and cherry shrimp so I always keep my tanks at 20ppm or less (usually less). In order to keep your biological filter intact...that is, to keep enough bacteria in there to break down all the ammonia should never do more than a 50% water change at one time. I also rarely change the filter on my HOB filters (the ones that hang on the back of the tank, my personal favorite) but instead rinse them out in a bowl of tank water before every partial water change. Note that carbon stays active for about a week but is not needed unless you have to remove medications or odors from the water. How does this help? Limiting the amount of available nitrates will limit the ability of the blue-green algae to re-grow.

To manage that blue-green algae I usually scrape the sides and decorations before doing a gravel vacuum. A thorough vacuum should be done at least every other week to keep all the excess food and waste out of the system.

Having a hard time keeping the nitrates under 20ppm? You may have to up the number of partial water changes, but if you find the parameters are going out of whack within a couple of days of changing the water you may have to take some additional steps to correct the problem in your tank. First, make sure your ammonia and nitrites are 0, if they're not you have bigger problems than blue-green algae and need to focus on getting your tank properly cycled again before continuing your quest to rid it of algae (though sides should still be scraped to keep the algae from reaching dangerous levels).

Next, keep an eye on your fish during feeding time. A fish's metabolism is such that any healthy fish will be pretty much perpetually hungry as they are opportunistic eaters that have adapted to the irregular sources of food in the wild so they should not be fed until they stop eating. Feed your fish the amount they'll eat in approximately two minutes for each feeding...I feed mine three times a day but twice is often sufficient. If you see quite a bit of food that doesn't get eaten or it takes significantly longer than two minutes for the fish to finish it reduce the amount they're fed.

If reducing food levels doesn't solve your nitrate problem do some research on your fish and your your aquarium overstocked? Bear in mind that larger types of fish such as most of the carnivores and goldfish have large stomachs and produce a lot of waste so they have particular space requirements and need filtration that will turn over the entire volume of aquarium water about ten times every hour (example: a 30G tank should have a filter that turns over 300 gallons every hour).

Finally, while it may be very difficult to get rid of the blue-green algae in your tank it can be quite easy to keep it within acceptable limits as long as you pay attention to the overall health of your tank. If you pay close attention to your water's parameters it is very possible to eradicate the algae or reduce it to levels you'll rarely even see...just keep up on that routine maintenance!


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    • profile image

      Rainkatt 6 years ago

      Is this available at pet stores? Like Petco?

      I want to try it with my 20 gal tank (1 10th of a tablet?) have lost both my placos, a goldfish and several smaller fish.

      Since I noticed white cottony spots on the fish I dropped in an effervesant tablet from the pet store.

      Turned the water blue for a few hours.

      I need a simpler way to handle this.

    • profile image

      jodi 7 years ago

      I had such a bad outbreak a few years ago! I hae since got it under control, this will help a lot of people!

    • Stoneage2010 profile image

      Stoneage2010 7 years ago

      Loved your Hub and rated it :-)

    • profile image

      peter (aquaman) 8 years ago

      Sorry Yoom, My tropical aquarium as stated 200 gallon (that’s about 750 litres) however I should have said my marine aquarium is 2800 litres. NOT 2000 gallon. If it was that big I could go for a dive in it.

      I do not know how a single ERYTHROMYCIN tablet would go in a much smaller aquarium. It may pay to be cautious.


      Peter (aquaman)

    • profile image

      peter (aquaman) 8 years ago

      To Yoom, the tablet I used for the 200 gallon Freshwater Tropical aquarium was a single ERYTHROMYCIN tablet. This is all that is needed to affect a cure. The great thing is its effect goes through everything. Gravel, filters and pipe work. Also you can wash any nets, testing equipment or buckets in the same water.

      A single ERYTHROMYCIN tablet also cured the problem of red algae in my 2000-gallon marine aquarium.


      Peter (aquaman)

    • profile image

      Yoom 8 years ago

      What kind of tablet did you put in your tank?

    • profile image

      Peter (aquaman) 8 years ago

      With regards to my statement in a previous submission "Hi, Well after some experimentation, it looks like I have finally found a simple fix to completely rid an aquarium of Black Algae".

      I was going to wait for a full month before letting you know the end result.

      Well no need. The cure has worked completely. All algae gone. All plants and fish 100% Aquarium conditions 100%

      So no need to add more to the subject.



    • profile image

      Peter (aquaman) 8 years ago

      Hi, Well after some experimentation, it looks like I have finally found a simple fix to completely rid an aquarium of Black Algae.

      It is literally an overnight cure. I have set up a trial in a 120 gallon aquarium that was infected with black or blue slime algae, and now there are no traces of Black Algae. This includes all pipe-work, plants, glass, gravel and filter.

      One tablet only did the trick. I am still monitoring the aquarium and keeping a log of everything. I will let you know later how it goes.

      It is the same cure I used to rid the marine aquarium of Red Algae. This never returned as the cure was not only in the aquarium but carried throughout the pipe-work and system.

      However in the Freshwater tropical it was a bit different. Conditions altered dramatically. The water turned like milk after 4 hours. But fish and plants came through ok and plants and fish are looking good. PH and GH are relatively stable.



    • profile image

      Peter (aquaman) 8 years ago


      If you should have an internal backdrop or scene glued to the inside rear of the aquarium, usually made from fiberglass or plastic. This to can also harbor algae spores behind in-between the glass and the backdrop, and it is with some difficulty, sometimes necessary to clean this type of in-tank backdrop.

      However what I did was to remove this type of backdrop from the inside of the aquarium. Then on the outside rear of the tank use the normal aquarium scene backdrop you can purchase from your Pet Shop.

      However if you prefer a rocky wall type scenario you can try the following. Take a piece of white polystyrene foam (about 2 to 3 inches thick). “The type they use in Esky’s” Cut it to the profile or size of the rear of the aquarium.

      Then with a gas flame or torch run it backward and forwards horizontally across the surface. Take care not to use too much heat, as it will melt right through the sheet.

      You will see the melting effect has created what appears to be a rocky surface. Then with some silicon, glue a strip of timber along the top edge. Then when the glue has dried, affix (with stainless screws) a couple of pieces of plastic angle, one at each end of the strip of timber. This will allow you to hang the backdrop on the back edge of the aquarium glass.

      Then you can paint this backdrop (before you install it) with ordinary outdoor “mission brown” house paint.

      Once your backdrop has dried you can simply hang it on the outside rear of your aquarium. It really looks effective. Unlike the stick on sheet type backdrops. Just like a rocky wall you will have shadows and light in a 3D effect.

      The mission brown color really brings out the color in the aquarium. The green of the plants, and colors of the fish look brilliant. The reflected brown also gives the watercolor appearance of a very slight brownish tinge, just like we find in nature.

      Keep the back inside of the glass clean and if for some reason you need to remove the backdrop just unhook it and lift off. This type of backdrop also looks great on a marine aquarium; I found the best color to paint on a marine aquarium is “Caribbean Blue”.


      Peter (Aquaman)

    • profile image

      Peter (aquaman) 8 years ago

      To Pirate FX

      Sorry in one paragraph my reply to your query on the use of ERYTHROMYCIN causing detrimental conditions in the aquarium.

      I wrote: Also in my freshwater aquarium I have a 20-gallon tank.

      I should have written:

      Also in my freshwater aquarium SERVICE ROOM I have a 20-gallon tank.


      Peter (aquaman)

    • profile image

      Peter (aquaman) 8 years ago

      Algae on plastic and silk plants can be a problem

      The method I use for cleaning plastic and silk plants.

      First mix twenty litres of water with half a cup of hydrochloric acid. This can be stored in a plastic drum when not in use.

      Important: Try to use the raw hydrochloric acid outdoors, wear eye protection and gloves. Try not to breathe the fumes of raw acid. After you have mixed it with water less fumes but still wear eye protection and gloves.

      Remove plastic plants and ornaments from aquarium. Place in bin, bucket or tub. Pour acid and water mixture over them. Sink plants and stir for 3 minutes. This will kill all algae spores.

      Remove from mixture. Hose off with water. Then return them to the aquarium. You do not have to worry about scrubbing or brushing off any algae, as over a few days in the aquarium the natural water movement from your filtration will remove the rest of the dead algae. Your plants will look like brand new.

      The water and acid mix can be stored in the sealed drum or container to be used over and over for years. Occasionally you can add a bit more water to replace any slight loss of mixture.

      You can even save some of the wash off water from the plants. As this is slightly acidic it can be kept in a bottle and if needed a small amount can be added to the aquarium to lower the PH to acid level.

      As long as you are careful with the amount you add it does not affect fish or plants in a detrimental way.

      When you use aquarium algae clear or killer, add it to the main re-seeders of algae. That is your filter. This will ensure all algae in the filter and pipe work receives the full force of the treatment and also mixes the algae fix with water before returning it to the aquarium.

      If you have sub-soil filter plates covered with gravel. Once every six months remove the top off the uplift pipes and one by one slide a siphon hose down the pipe. Start the siphon and this will suck any unwanted sludge or fine gravel from underneath the Gravel plates. This way you will ensure removing another source of nutriment for any algae in the aquarium.

      Finally if you can cut down any side or top daylight (Even indirect) on the aquarium (Your fluorescent lighting is OK) Try to limit your fluorescent lighting to 5 hours per day.

      You can however have a small 1 to 2 ft flouro on over the aquarium in the evenings. You can paint a few streaks of blue or green paint on the tube to give it the colour spectrum of moonlight. You can also purchase from your favourite aquarium shop, a bit more expensive blue or moonlight spectrum fluoro to do the same job. Your fish will love it and so will you.

      Another tip is ever tried to get to sleep when there is too much noise? Well think of your fish. During the evening place your ear to the glass side or front of the aquarium. You may hear a great rumbling noise from the many aquarium pumps and motors. See if you can address this noise problem. Timers are a great idea. If you can quieten the aquarium the fish will get a good nights rest and you can sleep happily in the thought your pets are enjoying a quiet few hours of darkness.


      Peter (aquaman)

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      Peter (aquaman) 8 years ago

      I have a 6 ft by 2 ft by 2 ft 3 inch deep tropical aquarium, also a 6 ft by 4 ft by 5 ft deep Marine aquarium. Both are algae free. I have never had to clean algae of the inside of the viewing glass in salt and freshwater, in well over a year.

      Both aquariums are built into walls with their own service rooms. Apart from the lighting, which are, Main lights (2 by 6ft) over each aquarium. On for 6 hours per day. During the evening hours a small 2 ft fluorescent linked to the same timer. for a few hours of evening viewing. Both of these fluoro tubes are the ordinary white but I simply painted a few strokes of green and blue paint on them to give a slight colour ting like moonlight.

      I use a very small amount Algae Clear, (the one they produce for Freshwater tropicals) NOT the marine one or the outside pool one! using 1 tenth of the recommended dose I mix it with a bit of aquarium water and instead of pouring it straight into the aquarium I pour it into the filter intake.

      For the small amount of volume in the filter this means the small amount of algae fix kills all the algae spores in the filter. Filters running constantly are great re-seeders of algae spores.

      Also I switch off all filters from 11 PM until about 8 AM the next day. No new water entering the filter for several hours mean the algae starves for nutriment, oxygen and CO2.

      Plus it is a great power saving. If you are worried about lack of any movement within the aquarium. There will be a slight movement when the heaters switch on or off. Warm water rising and cold water taking it's place.

      Conducting a bit of an exercise, I painted out the small window in each aquarium room. after several weeks algae in both aquariums died away. Then after some time I cleaned the paint of these small windows. After 1 week algae became apparent in both aquariums, especially the marine. I drew the conclusion that top and side access to long hours of any sort of natural light was the biggest factor in unwanted algae growth.

      A friend has an aquarium shop. All his tanks are lit all day with fluoro's. However no natural light comes into his main display room or little into his main shop.

      Another thing that is a problem in tropical aquariums is the sub-soil filter system; over time underneath the plates it can become clogged or blocked with sludge and mulm. The gravel especially the finer gravel can become blocked. Air stones can also deterorate with age, leading to practically little or no flow through your sub soil system. This leads to “Reversal” and dead pockets within the gravel. This is another great cause of unwanted algae, usually black. It gets on the leaves of plants. Slows or stops photosynthesis CO2 to Oxygen exchange.

      The way I have got round this problem is to simply extend the sub soil uplift pipes to about ¾ inch above the water surface. Then from the filter pump return I have a half-inch pipe leading to each upright with a small amount of water flowing with the aid of gravity down these upright pipes. This means detritus and other stuff does not clog the sub soil filter material. It also warms the lower area of the aquarium underneath the sub soil plates, so great for plants. You can also drop in plant food into these upright tubes. Several vertical tubes in your aquarium might seem a bit unsightly. Take a bit of aquarium backdrop scenario sheet, Cut it into several lengths and make a few tubes out of it. These can be simply slid over the upright pipes. (Great for Marine tanks too). If you use black plastic hose to carry the water to the filter uprights, it is hard to Asses if water is running along the tube to your filter. Simply insert a single garden dripper into the pipe. So if the water is dripping out it will indicate to you water is flowing to your filter.

      Another thing I did to starve nutrients to any potential algae. I fitted an extra 1-inch diameter pipe to the sub soil plate. This goes over the top and back of the aquarium to a large plastic tap. Then when I want to drain water for a change. I simply open this tap and water rushes down the drain. Taking with it any mulm or sludge under the sub soil plate. If the siphon on this pipe is lost I have simply glued in the top of the pipe a small air fitting. Then a length of air hose to a small airline tap. To regain siphon I simply close the main tap on the lower outside of the aquarium. Open the small tap on the airline, then suck on the airline until water appears in the clear plastic airline. Then close the small airline tap and open the main drain tap and water will once again flow freely down the drain. Just a word of warning. Try not to swallow any of the aquarium water or you might start to grow a set of gills and a funny tail.......

      Happy Aquariuming to you all

    • profile image

      Theresa 8 years ago

      Thank you for this detailed response. Unfortunately, I read this after I had dismantled my 28 gallon tank. I lost all my fish due to my ignorance. I think I'll start setting it back up today. I feel well informed on how to care for the tank now.

    • profile image

      Mike 9 years ago


      Your should be doing weekly water changes in your planted tank. Chances are monthly is just not enough to keep your habitat clean enough. Try doing a 50% water change and then follow up with 20% - 25% water changes weekly. This will reduce the possibilities of the BGA from coming back in the future, however you may have to resort to medication to get "rid" of your current BGA situation.

      It can be removed mechanically but will return quickly if the water quality is not fixed. It can be treated with Erythromycin phosphate, but this might effect the nitrifying bacteria in the gravel and filter. When the BGA gets killed by the algaecide it will start to rot and through that process it will reduce Oxygen levels in the tank. Since the nitrifying bacteria needs O2 to transfer ammonia/nitrItes into nitrAtes the nitrifying process will slow down. If algaecide is used, make sure to test the ammonia/nitrite levels. Remove all the visible algae to prevent it from rotting in side the tank.

      Some aquarists use the black-out method, where black bags are wrapped around the tank for 4 days and held in complete darkness. It is advisable to raise NO3 levels to 10-20ppm before starting the black-out period. All visible algae should be vacuumed before black-out and after the black out.Egeria densa (Elodea) and Ceratophyllum demersum are good plants to have in a tank. These plants secretes antibiotic substances which can help prevent Blue-Green Algae. Establishing, lots of healthy, fast-growing plants from the day you start the tank + dosing the nitrAte levels to maintain 10-20ppm (in planted aquariums) and vacuuming the gravel (in non-planted ones), is the best way to prevent this "algae". The BGA can be found in aquariums with very low nitrates because it can fix atmospheric nitrogen. BGA seem not to like very low pH and high CO2 levels. BGA doesn't prefer strong water currents. Excess organic loading is the real cause in many cases. Try removing decaying plant material and prune old leaves that are leaking organic nutrients back into the water column.

    • profile image

      fishmom 9 years ago

      I have a 55 gallon freshwater aquarium with 3 angels, 2 crebenza, 6 platys and a few otocinclus and LOTS of blue-green algae. Try to limit feeding and do water changes about monthly. I have quite a few live plants and the stuff just gunks all over them. What else can I do?

      Other issue: my crebenzas just hatched a brood of reluctant to do a water change right now and disturb them.

    • livelonger profile image

      Jason Menayan 10 years ago from San Francisco


      We only have 2. Dare to dream. :)

    • wychic profile image

      Rebecca Mikulin 10 years ago from Sheridan, Wyoming

      Glad I could be of help :D. I'm not certain how I fell into the fishkeeping business but I just kept going with it (that was many years ago), now it's my favorite obsession and I'm in the process of building a new fish house for 400 tanks :)

    • livelonger profile image

      Jason Menayan 10 years ago from San Francisco

      What a great thorough response! By far the best thing out there on this subject.

      I think we did try a greenish medicine once - and yes, it did seem to kill our snails :(

      It's strange because one of our tanks only has that filamentous green hair algae (which is easy to gather and get rid of), and never the slimy blue-green stuff. And the other has only the slimy blue-green stuff which grows out of control.

      We'll try restricting the feeding and see if that helps. Thanks again!


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