My aquarium has live plants with a betta, hatchet fish, barbs, a cory, and a Siamese algae eater. Black algae is growing on the decorations and glass. I scraped it off the glass. What else should I do to keep it from coming back?

My tank is not next to a window and the light is on for 8 hours a day.

  • Are you talking about cyanobacteria (it goes by many names; most of which contain the word "algae")? It looks like a dark, slimy algae that scrapes off in sheets. If so, that requires a bit more to get rid of than algae
    – Gary
    Commented Nov 14, 2014 at 20:05

5 Answers 5


The first thing you want to do to battle algae is to remove excessive light. The worst is when sunlight reaches the tank. Algae can grow in very little light, so given sunlight (or even a tank light that's on for more than 12 hours), algae will be able to grow even faster. It's different for each tank, some of mine I can keep lit for almost 12 hours, others for only about 8.

Once you have your light regulated, you're going to want to take some extra steps to reduce algae growth. This all comes down to removing the nutrients that the algae uses to grow.

  • Scrape any algae you can off the glass. By far the best option is to get in there with an algae brush and get rid of it by hand. The more algae you scrape away, the less is able to grow later on. Make sure to do a small water change after this to remove the algae you just scraped free.
  • Check the water you use for phosphates; they act as a fertilizer for algae. If you can, get some RO water from a nearby pet store, or jugs of distilled water from a store/vending machine. You can put phosphate remover media in your tank filter and/or if you are able to, you can even put an RO filter in your sink.
  • Clean the gravel. Any waste from your fish, including leftover food, is going to be sitting in the gravel. That waste is food for algae, so remove it and algae can't eat. Gravel vacuums are a huge help with this.
  • On that note, make sure not to overfeed your fish so that there isn't any leftover food. You should only feed your fish what they will eat in a 5 minute timespan.
  • I've used API Algaefix on an algae bloom before and it worked. Although, I found that I would keep having to use it which I didn't like. It might work differently on black algae, but I wouldn't rely on it.
  • I've had the best luck with goldfish (the 15 cent feeder goldfish you can get at the store) eating all types of algae including string algae and quickly. Once they reached 3-4 inches in length 2-3 of them seem to clear a 40 gallon aquarium overnight. The only caveat is you'll need a place for your new pet goldfish. Once they grow large enough they'll start eating your plants.

There are some things you can do to proactively rid your tank of algae. If you think you have the algae taken care of for the moment, but it might come back, these are some things to think about.

  • Keep using phosphate removers/RO water as I mentioned above.
  • Add more plants to the tank. The more plants you have in the tank, the more competition there is for the resources. Even small plants can help.
  • It works best if you have several broad-leafed plants in your tank, but adding supplements like Flourish Excel works by helping your plants absorb nutrients better, helping to starve the algae.
  • I've used a UV filter to get rid of algae blooms that left my tank cloudy. While it won't get rid of algae that's attached it will kill any that's floating freely in the tank. So far the only algae it hasn't been able to get rid of is string algae.
  • Hydrogen Peroxide will kill algae. And will diffuse into water and oxygen after 24 hours. As part of getting rid of hair algae in my tank I quarantined my plants and dosed them with a few tablespoons for two days. If you're doing it in a tank with fish in it. It's a ratio of 1 part hydrogen peroxide to 150 parts water. (reference)
  • You can add algae eaters to the tank. But keep in mind that they'll only eat so much per day. So it's a matter of having a higher ratio of algae eaters to algae that they can eat.

    • Probably the one of the most common algae eaters are snails, nerite snails seem to be the most popular.
    • Shrimp are possible depending on the fish you have.
    • Plecostomus are a good option, but go for a bristlenose if you don't have a tank larger than 40 gallons.
    • Otocinclus catfish are about the best fish for algae in my opinion; though they are quite small and sensitive to water quality.
    • My farlowella catfish seem to eat everything but string algae.
    • Other types of catfish won't help with algae, but should help take care of leftover food in the gravel that the algae uses for food.
    • If you have a larger tank, Siamese Algae Eaters and Flying Foxes are good algae eaters. They get around 6 inches in length which is why they need some space.

On a side note: Chinese Algae Eaters, despite their name, are terrible for taking care of algae; they're too lazy, and sometimes eat away the slime coat from other fish. They are unfortunately confused, and sometimes sold as Siamese Algae Eaters. Flying Foxes also get sold as Siamese Algae Eaters, but that isn't as bad of a trade-down.

  • This answer is spot-on. If it's blackbeard algae, you may be hard-pressed to find a fish that will eat it, though. Commented Jan 6, 2014 at 15:36
  • @Raystafarian Yeah, for freshwater algae, if it's not green, it's generally safe to assume nothing wants to eat it. Maybe a true Siamese Algae Eater or a Flying Fox, but I've heard mixed reports.
    – Spidercat
    Commented Jan 6, 2014 at 15:55
  • Nerite snails have the most varied diet of algae eaters, from what I understand. That's a pretty accurate general rule, though. Commented Jan 6, 2014 at 16:23

If you currently have an algae problem, the main thing to do is identify the cause of the algae. Usually the cause is an imbalance in the tank, usually it's due to excess light or excess nutrients. This is something you'll need to balance so that you stop battling the algae.

However, as this question is "how do I get rid of algae," I'll outline the steps -

  1. Scrape off the excess algae on the glass and decor with a razor blade. If you have acrylic, use a magic eraser.
  2. When you scrape off algae, it may die. This will cause water issues, so also perform a water change.
  3. Since you're changing the water, remove waste by doing a gravel vacuum or something similar.
  4. As you're doing maintenance, might as well clean out the filter in some tank water so it operates a little more cleanly.
  5. Black out the tank. No light at all, this will kill the algae. If you have other plants, you can't black out for too long, but they will survive better than the algae. You can also cover the tank with something to block out ambient light.
  6. If you dose ferts or run co2, you can try reducing or stopping that. Your plants may melt in which case they will fail to out-compete the algae, so pay close attention.
  7. If need be, run some carbon in your filter or even dose some seachem excel directly on algae spots.
  8. Repeat as necessary. More water changes and gravel vacs has never hurt.

Performing these steps should kill the algae, but remember the best key is to find out what is out of balance and prevent the algae from coming back!


I use Seachem Flourish Excel at 1.5X for a week and then at the recommended strength until it's gone. All up, about 2 weeks. I have had two outcomes from this method. 1. The algae disappears or is reduced to pin head sized dots on some of the plants. 2. The algae goes from black to blonde and then molts. I don't know what factors have influenced these quite different outcomes. Good Luck.


I dosed my tank with "flourish" at 3X rated dosage 20 hours ago in an attempt to knock the black algae back on my amazon swords (This method has worked in the past at 2X normal dosage so I thought I would up the ante a little). For some reason my mollies and muppies (deliberate x breeds) are now all over the black algae and vigorously eating it. This has never happened before, the fish normally ignore the black algae in favor of their regular food. At the rate they are going the algae will be gone in a day or two.

Mollies eating the black algae(sorry about photo quality)

  • 1
    It would greatly improve your answer if you had experience with this for multiple times, or another case from somebody else who also got this result. A one time miracle, as it is now, is not really making it a recommendable answer. Commented Sep 6, 2014 at 7:09
  • I've recently heard this suggested as a use while I was looking up plant supplements. The idea is that it helps the larger plants absorb nutrients, which will starve the algae. Another answer here seems to support it too.
    – Spidercat
    Commented Nov 25, 2015 at 21:14

It's not impossible nor even slightly hard to remove black algae. I've had it in all of my tanks mainly because of scheduling. I leave my aquariums running 24-7. I've even bought used tanks with that are covered in it.

I fill the tank, let it cycle and drop in some of my snails. I'm aware most people hate snails, but they're actually a really great cleaning crew. For instance, I picked up a 20 gallon tank a few moths ago. It was loaded with black algae. It took the snails about 7 to 10 days to clear the tank and I'm now using it for gourami.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.