Freshwater Aquarium Algae: How to Identify, Treat & Prevent

So, you have algae? There’s no shame in that, we all have algae. Pull up a chair and let’s talk it through.

I wish I could tell you that I am able to give you a cool, logical discussion on algae. But I am a little obsessive and it really, really bugs me to see my tanks get a visit from the algae monster. In truth, though, it is really a matter of when there is too much algae or the wrong sort of algae.

A guide to help you with freshwater algae identification treatment and prevention

Obsessive or not we have to accept that as aquarium owners we are bound to play host to this stuff most of the time and the sensible approach is to learn to recognise whether our algae is something we can live with or something that if left unchecked will ruin our tank.

There are five things to look at here:

  • Identify your algae – a list of algae types with specific cures where they are known.
  • General recipe to help avoid algae.
  • Tools to help fight algae.
  • Algae eating critters.
  • Chemical cures - a warning and a promise

Avoidance of algae is really the most important, as if you don’t cure the reasons algae are doing too well in your aquarium you can pull it, scrape it or poison it as much as you want and it will still come back.


Types of Freshwater Algae

First, though, let’s get to know the enemies. It is so important that you identify what algae is your main problem as it will give you clues as to how to deal with it.

I have tried to list all the common types, but I’m sure you have examples that I’ve missed. I would really like to get feedback on this, especially if you can send us pictures of what you are dealing with.

1. Brown Algae (diatoms)

This starts as a light brown dusting but can build to be quite a thick ugly layer. Diatoms are plants but they can exist with very little light. They are often associated with new tanks, high silicates and high phosphate levels.

If you have this in the first month or two or your tank set up, wipe it off hard surfaces, clean your gravel, change your water a little more often and it may well clear up. Many of the algae eaters we discuss later love this stuff.

This is an algae you can normally handle. Many of the algae eating critter listed later will go to town on diatoms. You can resins to absorb phosphates or silicates but try the low-tech method first.

2. Blue-Green Algae or BGA (cyanobacteria)

Technically this is not an algae but I doubt that will make you feel any happier. Blue-Green Algae is a bacteria but it can photosynthesize like a plant. It grows in thin layers, that as they get thicker can be removed in slimy, smelly sheets. The most common variety in freshwater is a really vivid green but it can also be reddish or brown.

This is a BAD guy it can take over your tank and it is hard to get rid of. There is an antibiotic cure but this will not be available in all countries and is perhaps a bit extreme, it will also harm your filter bacteria. We have a complete guide to treat cyanobacteria, have a look at it!

Blackouts are a popular treatment among planted tank enthusiasts. This means excluding ALL light from your aquarium for a certain period, often by taping black bin bags over anywhere light can penetrate.

The blackout method

  • Remove as much of the algae as possible.
  • Do a 50% water change and clean your filters thoroughly.
  • Blackout your tank for 96 hours, turn off any carbon dioxide if you use it and don’t feed your fish.
  • Uncover your tank, repeat the 50% water change, clean your tank and filters routine.

I am particularly interested to hear from readers who got shot of this pest without using antibiotics. We need to know your experiences.

3. Green Hair Algae (GHA), Thread Algae

Green hair algae comes in many forms. Small clumps of short hair, masses of strands inches long which mat together or just occasional strands on the edges of plants and on rocks. Left alone, it will spread to become a problem.

This is my personal battle right now. In my month old planted tank, I have the first traces of green hair algae starting on my Echinodorus , I remove them, they come back. It’s annoying stuff.

I advise what I’m doing, 2 water changes a week to make sure my water is clean. Keeping my fish feeding low and physically removing as much algae as I can. If there is a lot of the stuff a toothbrush is great to twizzle around and wind it up.

It is not bad enough that I’d do anything more drastic. Some plants are particularly affected and since they are still growing strongly I prune off the worst leaves.

One thing I have read about but not tried yet is to increase the nitrate level. The opposite of what I’m normally aiming for. If anyone has successfully tried this method, let me know.

4. Staghorn Algae

Staghorn Algae grows in strands but unlike Green Hair Algae it branches like a deer’s antlers. It is grey-green and will grow on most surfaces. The most quoted cures are physical removal, less lighting, cleaner water and more water movement.

5. Green Water

Green water is caused not by the water changing color but by millions of single-celled algae floating in your water. The good news is that one way or another you can normally cure this problem.

The ‘bloom’ of algae is triggered by an abundance of light and food. So prevention will mean the normal water hygiene steps of water changing, good filtration and avoiding overfeeding your fish. Just doing that may be enough to clear your water over time but you can speed things up with a Ultra-Violet steriliser or a Diatomaceous filter.

UV sterilisers are now available as small internal units that go into your aquarium. The steriliser kills the algae, the diatom filter (most units are external filters) is so fine it sieves them out. Neither option is cheap.

Blackouts similar to that suggested above for Blue-Green algae have worked for some people but not for others.

I found this interesting post by a guy who did a little experiment which showed that plants were a solution, out-competing the algae for food and light.

These product links are here by way of examples, I have not tested them. Do your own research.

6. Green Spot Algae (GSA)

A dark green algae which grows in small, flat spots. It normally keeps to your aquarium glass but can spread to the leaves of plants. The ‘special’ feature of this algae is that it is tough and is not normally cleared well by magnetic algae cleaners. A carefully used metal bladed algae cleaner is the most effective way to remove them. If you have an acrylic tank you much us a plastic bladed cleaner, short handled ones let you apply more pressure. See my section on tools below.

If you have a lot on your plants it is worth pruning off the worst affected leaves.

This is an algae that enjoys a lot of illumination so consider reducing the hours you have your lights on. I have not been able to find any reliable way to get rid of this algae but if you tank is suitable for them, Nerite snails are said to help keep it down.

7. Green Dust Algae (GDA)

This algae has a very disturbing ability. It looks a bit like the normal green algae growing on our tank’s glass. You wipe it off and go away happy but when you come back in a couple of hours the film has come back!

GDA is effectively able to swim and then resettle and start growing again. GDA seems to only like growing on smooth surfaces and doesn’t bother plants or rockwork. No-one seems very clear on what causes it but I have a great link to Tom Barr’s cure.

In short, it is suggested you just let it grow and grow for up to 2-3 weeks until it forms thicker patches which fall off the glass. At that point, you step in and do a big clean out.

8. Black Brush Algae (BBA)

BBA grows in dark tufts, often on the leaves of slow growing plants but also on tank equipment such as filter outlets. It is hard to remove the tufts from plants and if they are badly affected you are better or discarding them.

Equipment can be treated with bleach if they are very carefully washed before returning to your tank. Some suggest that fluctuating carbon dioxide levels in planted tanks encourage this menace.

Siamese Algae Eaters are said to snack on this stuff.

I have a link here with a detailed suggestion of BBA remedies. Do note that some are very chemical in nature and could be dangerous to your fish if done incorrectly. I put it here in case you are fighting a losing battle and want to know all your options.

9. Water Surface Algae

In tanks with sluggish water movement, an oily film can develop on the water surface. On occasion, this can attract an algae which grows in it.

I had this problem in the early days of one tank, my solution was to just manually skim the surface by pushing a glass just a fraction below the water and drawing off the top layer.

You can also increase the surface agitation of your water by adjusting filter outflows or adding powerheads.

In my tank, the problem went away after a couple of weeks. You can buy an electric power skimmer if the problem is ongoing.

Recipe of tank maintenance to keep algae to minimum

I think there are two real types of algae issues. Those in planted tanks and those in tanks where the fish are the main feature. The reason things are different is because planted tanks tend to have maxed out lighting, carbon dioxide, and fertilisers. This solves some algae problems and causes some others.

I have references at the end which give great info on planted tank algae management.

In a fish dominated tank things are simpler in a way and you can focus on reducing algae in several ways:

Bob's Algae Checklist

  • Keeping the number of hours you have your lights on low. More than 9 hours will encourage algae.
  • If you’re planning a tank, put it out of the way of direct sunlight. One of my tanks gets 30 minutes of sun a day, if I don’t close the shades I will get extra algae.
  • Change at least 25% of your water once a week, vacuum any dirty gravel as you do.
  • Clean your filters once a month (more in a heavily populated tank).
  • Always dechlorinate new water added to the tank.
  • Feed just what your fish really need.
  • Jump on small algae problems before they get out of hand.
  • Keep the fish population well below recommended limits.
  • Try not to disturb your gravel unless you are cleaning it, there’s a lot of much in there that can trigger an outbreak.

If you follow these steps, at least when you get a bad algae outbreak you can know it wasn’t through neglect. Well maintained tanks get algae but poorly looked after ones get a lot more.

Algae Fighting Tools

You really don’t want to let algae get too established anywhere so it is sensible to

  • Clean your glass weekly
  • Vacuum your gravel as you water change
  • Take out and clean rocks with even a hint of the really bad algae types
  • Prune badly affected leaves of plants and discard the whole plant if its beyond hope

These are the tools I use. Other brands might be available as well…

Algae Eating Critters

I believe this is nearly your last step in the fight. Not because they don’t make a useful contribution but because most of the other cures can be used whatever fish you are keeping. However, whenever you add new creatures to your tank you should work out if they match your existing fish, your water, your tank size etc.

Another thing to remember is that different critters eat different algae and I don’t know of any that eat everything. I have a lot of algae eating species in my planted tank but I still have algae and they completely ignore the Green Hair Algae that bothers me most. I know in some tanks the same animals eat the stuff.

My point is your algae eaters can be a big help but not a cure and they must fit your system.

Here is a list of some contenders.

1. Siamese Algae Eaters (SAE)

A good algae eater, that targets a lot of algae types including BBA. It does get to six inches and prefers company so really only for larger tanks. SAE like water movement and clean water.

There is a big problem with identification, reportedly a lot of fish are miss-sold as SAE, and the fake fish you get might get could be quite aggressive. Read up the subject, this link is a good start. 

2. Bristlenose Pleco, Ancistrus

Hardworking algae eaters especially when young. Most Ancistrus sp. only grow to around six inches. They can be kept singly and are therefore a good choice for a mid-sized tank. They are also peaceable.

Not much downside, mine ate fewer algae and more fish food as he aged.

We have a more detailed pleco catfish article, let me know if you find it useful!

3. Rubber Nose Pleco, Bulldog Pleco

I have heard mixed reports on how much algae these fish eat. They do stay at around 6” and don’t have any other big issues.

I would love to hear from you if you have kept these guys. Did they work hard on the algae?

4. Otocinclus, Otos

Otocinclus are great eaters of brown and some green algae, good for planted tanks with quiet inhabitants. Need super clean water and to be kept in a shoal. Not my pick for an average community.

5. Hillstream Loaches

Small flattened fish shaped a little like a guitar. Eat algae but need very clean fast moving water. Not really a normal community fish.

6. Twig Catfish

Amazingly camouflaged and fascinating fish but could only be kept with small peaceful companions in a rather specialist tank with lots of bogwood and super clean water.

7. Chinese Algae Eaters

I would stay away from Chinese Algae Eaters unless you have a very boisterous community of tough fish. They get aggressive and territorial with age.

8. Nerite Snails

Really a useful addition to many tanks. Eats a lot of algae, including green spot algae. Cannot breed in freshwater but is happy living in it if it is neutral to alkaline and fairly hard.

Quite a lot of varieties to choose from. My favourites are the horned guys.

If you want to know more have a look at our freshwater snail species profile.

9. Malaysian Trumpet Snails

These are on my “be warned” list. Some people like trumpet snails but they really eat more debris and old fish food than algae.

They hide in the gravel during the day and come out at night to feed. I can see they might be useful in keeping things clean but the problem is they breed very well.

I read a great comment about someone’s tank being like “The Return of The Living Dead” at lights out. They are very hard to get rid of.

10. Apple Snails

Ok, but they do eat plants as well as algae. They really grow too large (up to 6”) to be able to clean many nooks and crannies.

If they start breeding, numbers can get out of hand.

11. Shrimp

This really needs a whole new article. There are many shrimp types with different needs. Some are fussy about water, some less so. Most are really small and very vulnerable to attack so they can only be kept with small, non-predatory fish.

Amano shrimp are a good place to start researching if you have a peaceful planted tank. They will eat some algae, but not hoover glass.

Chemical Cures

This will be the topic of another article. The first reason is that there is so much scope for things to go wrong and so many warnings that I can’t just fit it all in a paragraph. The second is that they do nothing to solve the underlying problem that caused your algae.

I will cover shop bought algaecides, hydrogen peroxide, organic carbon overdose and nutrient balance. If you do some of these wrong and you can kill all your fish and plants.

Closing Thoughts

I really wish you luck with the algae. I know it can hit the most fastidious fish keepers.

Let me know what tips helped you and what has been a flop in the comments below!

Further Reading