Algae: one of the most complicated topics in the keeping a beautiful planted aquarium. Algae is often the reason why many people give up on creating their dream tank as it can devastate the aesthetic and completely smother out their plants. After a decade of experience, there are a few algae management techniques that I've discovered which address the fundamental aspects of a successful planted tank in order to create a pristine environment without the use of harmful algaecides.
Before I delve into this topic, I’d just like to state that the scientific knowledge and research on the topic of freshwater algae, specifically the types in the aquarium, are scarce. Most of my knowledge of algae comes from over a decade of experience and from planted tank experts such as Tom Barr and Dennis Wong. I’ve helped countless clients solve their algae issues in their own tank so I have verified most of this information through trial and error, as well as some experiments I’ve conducted over the years.
In order to get rid of algae, it is first important to understand the organism itself.
Cause of Algae: Ammonia, CO2, and an imbalance of nutrients
All a lgae species are originally present in the water through spores. The adult algae is what we actually are able to see in our aquariums and this is what we strive to get rid of.
So why does algae appear? It appears because something is wrong with the plants. Think of algae as an indicator for everything wrong with the plants themselves. If there is algae in the tank, the plants are not in ideal health. Either there are too few nutrients (very common), not enough CO2 or fluctuating levels (this is the main issue), an imbalance in nutrient quantities, or there is a high amount of ammonia in the water.
When plants don't get the nutrients they need (macros, micros, and CO2), then algae will appear because the plants will start to die. When they die, they release ammonia and other chemicals that trigger the algae spores to become adult organisms. Algae is an opportunistic organism when co-existing with with aquatic plants. It appears when the aquatic plants are weakening which leads me to a theory of allelopathy in aquatic plants.
Algae is highly opportunistic and can use a variety of nutrients and CO2, etc. So what gives plants the edge over algae? It has to be some form of chemical warfare/inhibition also known as allelopathy. There is NO way that plants “outcompete” algae since I’ve had adult algae living in water with 0 nitrate 0 nitrite 0 ammonia with a minuscule bioload and minimal feeding.
The reason why CO2 injection is almost vital in combo with fertilizer is to facilitate tremendously lush and vibrant growth which gives the plant enough energy to create allelopathic chemicals. That’s also why a dense plant mass in the beginning is vital for all tanks so that there is a high enough concentration of allelopathic chemicals in the water. It might also explain why a tank full of slow growers is more likely to get algae as the overall metabolic rate is much slower so less allelopathic chemicals are produced compared to crazy fast growers like Anacharis and hornwort. The only reason why we use CO2 injection is because it allows the plants to reach their full potential and truly thrive as they do in the wild. It’s reasonable that the last function that plant cares about is producing the allelopathic chemicals to fight off algae. The primary objective is to simply sustain itself and propagate. Ensuring minimal competition (algae and other plants), comes last so they only produce these allelopathic chemicals when they’re truly happy which is when they’re growing in CO2 and nutrient rich water.
Note: Allelopathy has not been completely proven in aquatic ecosystems yet and is simply a theory of mine that applies to the planted tank. However, in the context of a planted aquarium, it makes sense.
Algae is NOT caused by "excess" nitrates and phosphates. Otherwise many aquarists such as myself would have an algae farm when they dose 60+ppm of nitrate per week and dose 15+ppm phosphate per week. Plants do not "outcompete" algae as algae is one of the most primitive organisms around. They need very little nutrient quantities to survive and grow. Limiting nutrients will hurt your plants more than the algae. The only nutrient that "causes" algae is ammonia because it allows the spores to become adults. That is why dosing urea or ammonium nitrate might be risky in the planted aquarium, as a straight source of ammonia in a high enough quantity may spark an algae bloom since there is enough ammonia available to allow the algae spores to become adults.
Note: I am not stating that light "causes" algae. This is because aquatic plants can be grown in the strongest source of light (direct sunlight) without algae, so it goes to show that if you have high enough CO2 and balanced/sufficient fertilizer levels, you won't have an issue with algae even with super strong light sources.
Algae Treatment and Cure:
The first treatment is simply manual removal. I do not condone the use of algaecides unless it is truly the last option or the issues in the tank have been fixed but the adult algae needs to be removed. Algaecides like hydrogen peroxide and excel when used in high quantities can pose an issue for the beneficial bacteria colonies in the tank.
The second treatment option is the use of a UV sterilizer. This is really to be used after the algae has been manually removed or in the case of green water. The UV sterilizer will kill the algal spores or adult algae floating in the water column.
Here's the cure for most algae:
Start fertilizing your aquariums with a complete formulation of nutrients including nitrogen, phosphate, potassium, iron, magnesium, calcium, and the rest of the trace elements (zinc, boron, molybdenum, etc). Thrive by NilocG is a great option for beginners and dry fertilizers are a better option for the more advanced aquarists or those with large aquariums.
Inject CO2 into the water. If you're already doing this and still have algae issues, your CO2 levels aren't high enough. Disregard your drop checker or your pH/kH tests and just watch your livestock. Bump the CO2 high enough until the point that your fish become slightly stressed and start to gasp at the surface for air. Then lower it just a tad down. Lower it just slightly so that the fish return to their normal state, but the CO2 levels aren't lowered significantly. This threshold CO2 level between stress and no stress is where you want the CO2 level to be at permanently. This level is the perfect concentration of around 20-30ppm CO2 which is ideal for aquatic plant growth.
Increase your filtration. Over filtration is often better than under filtration. Try to use established filter media if you have any. Increased filtration equates to less ammonia and allows for a larger margin of error in the aquarium.
For those who do NOT want to start CO2 injection:
Your journey to an algae free tank just got 10x more difficult. Lowtech tanks are often famed for being "low maintenance" but that is anything but the truth. Having an algae free lowtech tank requires optimal and fragile balance. I'm not saying it isn't possible but it is way more difficult than it leads on to be. In order to have an algae free lowtech tank, do a split photoperiod and fertilize sufficiently. Keep the photoperiod at around 8-9 hours and split it 50/50 with a 2 or 3 hour rest period in between. Thrive is perfect for lowtech tanks. Experiment with lighting levels. Some lights are way too powerful for a lowtech tank. Too much light without enough CO2 and fertilizer will cause an algae disaster. Get a high quality 6500K light for a lowtech aquarium. 6500K CFL grow bulbs often are great for lowtech aquariums because they aren't too powerful, they're cheap, and they last quite long. Make you sure you keep up with fertilization and filtration.
Pressurized CO2 does not have to be complicated or expensive. It is very easy to use and makes a HUGE difference.
Here are is a specific list of algae I've encountered (ranked from easiest first to most difficult last) and how I got rid of them:
Diatoms are simply caused by new tanks. A lack of sufficiently established media and high light often paired with no ferts or CO2 will cause diatoms. Diatoms are also common in tanks with sand although there has been compelling evidence as well that the silicates in the sand do not trigger diatom explosions. The cure to diatoms is simply to wait it out. Let your filter media become more established and keep fertilizing your plants sufficiently. Keep up with water changes and within a few weeks or days, the diatoms will be gone.
2) Green Water Algae:
If you notice your water starting to get cloudy or developing a green tint, act immediately as this is green water algae. This is a free floating algae that makes the water a dark green if not addressed immediately. It is often caused by excess ammonia in the water due to plants decaying and/or overfeeding food. Large water changes, increasing filtration, and adding UV sterilization is the best way to get rid of this algae. Adding UV sterilizers are EXTREMELY effective and it is the only way I've quickly gotten rid of this algae without having to perform multiple large water changes over the course of many weeks.
3) Slimy Filamentous Hair Algae:
This is the most primal sort of hair algae. It is caused by excessive ammonia in the water. To cure this, manually remove it and then increase filtration and flow. Also increase fertilization and CO2 to be on the safe side as well. This is actually very easy to get rid of and often disappears within days of fixing the necessary issues.
4) Green Spot Algae:
Getting rid of this algae is very easy. It is usually due to a lack of CO2 and phosphates. Simply increase phosphate dosing to a healthy 3-5ppm per dose and if that doesn't work within a few weeks, increase the CO2 as well. The only reason why this can be slightly difficult is that it doesn't go away within days or hours like some of the other algae, and can be a bit persistent.
5) Black Beard Algae and Staghorn:
This algae is often notorious for getting rid of, but that is often because treatment is often attempted incorrectly. Most people blast this algae with excel and hydrogen peroxide but it will come back because the root issue hasn't been addressed. This algae is due only to fluctuating and low CO2 levels. Very easy to fix. Simply fix your CO2 levels (increase it or get a pressurized system) and the algae will turn red within hours and disappear within days. This treatment works for both staghorn and BBA. If you have BBA, you most likely also have staghorn algae and vice versa. The two show up when there isn't enough CO2 or fluctuating levels.
6) Green Hair Algae:
The good old hair algae. It isn't coarse, but it isn't exactly soft to the touch either. It literally looks and feels like hair. This algae is the culmination of everything out of balance in the tank and it is difficult to pinpoint exactly what caused unlike BBA and staghorn algae. The best thing to do with this algae is to remove as much as you can manually and then upgrade or clean the filters, increase CO2. Keep up with water changes and eventually this algae will go away. Once it goes away, it generally doesn't come back as long as good conditions are maintained.
This is a very difficult algae to eradicate, but not impossible. It is caused by a lack of CO2, but it tends to be very hardy unlike BBA and staghorn algae. Manual removal is often best and then the increase CO2 should kill the remaining algae.
This is by far THE WORST algae I've ever dealt with. Even after increasing filtration, increasing CO2, and really dialing in on my fertilization regimen, I still battled this algae for a long time. The problem with it wasn't that it was growing super thick and suffocating plants, but that it's growth was so wispy and sticky that it was definitely visibly tangled in the plant, but it was impossible to try and remove. Spirogyra often doesn't grow thick and only establishes itself in tanks that have been set up for a long time. This is likely the last type of algae your tank will experience before it is truly algae free. Even after fixing everything, it may still persist, just little strands tangled up in the plants. It won't grow too much more, but it will be there. So what's the cure here? Time and patience. It is a long lived algae that is very persistent. But if conditions have really been made better, the algae will eventually disappear and stop growing. It takes a LONG time though.
Myth 1: "Excess Nutrients" Cause Algae
This is by far the biggest myth when it comes to algae management in the aquarium. When people talk about "excess nutrients", they are usually referring to nitrates and phosphates.
Excess nutrients do NOT cause algae of any form including hair algae, BBA, green spot algae, and green water algae. There is only one nutrient/type of nutrient that causes algae and that is ammonia. Most aquarium fertilizers do not contain ammonia, but ammonia is produced by organic decay AKA uneaten fish food, fish waste, dying plant matter, detritus, dead fish, etc.
Nitrates and phosphates do NOT cause algae. In my tanks that are outdoors in full, direct sunlight, I dump in phosphates and nitrates plentifully. I'm talking around 90ppm nitrate per week (15ppm dosed per day) and 15ppm phosphate per week (5ppm dosed 3x a week). According to the idea that "excess" nitrates and phosphates cause algae, I should be having an algae disaster right now. But I'm not. The theory that nitrates and phosphates cause algae is just simply outdated when applied to freshwater algae. Yes, adult algae consumes nitrate and phosphates but they don't last long in their life cycle. The spores need ammonia to manifest into adults.
The only time when "excess nutrients" causes algae is when there is an extremely high amount dosed to the point where one nutrient blocks the other. For example, high amounts of potassium may block calcium uptake. When the uptake of another nutrient is inhibited, the plant health will decline and will release ammonia as it dies, therefore leading to algae as a chain reaction. Often, this is an issue when fertilizers are immensely overdosed and there is a lack of water changes to reset the nutrient levels in the water.
Myth 2: Fertilizers Cause Algae
Going off of the previous point, fertilizers do not cause algae. It is common for someone experiencing algae to cut off their fertilizers in an attempt to "starve" the algae. Your plants will starve far sooner than the algae will. In fact, the algae will feed off of the starving plants as they inevitably decay and release ammonia. Preventing algae equates to dosing complete macro and micro fertilizers so that your plants are healthy and will not decay.
Myth 3: Iron Causes Algae
This was definitely a rarer myth, but comes up enough for me to feel the need to mention it. The logic I've seen to justify this claim goes something like: "Iron is very important for chlorophyll production so therefore it is also responsible for causing algae". There have been some cases documented on some planted tank forums where adding lots of algae apparently "causes" the algae. When the iron is lowered or removed, the algae subsequently goes away. One thing to acknowledge about these cases is that these tanks are often new. When the person stops iron dosing, the tank becomes algae free within a few weeks. This could be merely coincidental, as algae subsides as the plants adapt to the tank's specific conditions and the CO2 injection stabilizes and reaches its ideal amount for that particular tank. In newer tanks, a few weeks is a very long time as many changes and boom/bust cycles of algae tend to manifest within a short period of time. I've dosed tons and tons of iron without any algae growth. Anywhere from 0.3ppm to 4ppm every other day. 4ppm is very extreme and really is just a waste of iron. I aim for 0.5ppm 3x a week.
Myth 4: Excel "Cures" Algae
This is another myth that is commonly parroted. Excel is simply a diluted solution of a biocide that is often marketed as "liquid" CO2. Excel is not liquid CO2 and does not provide a noticeable amount of carbon for aquarium plants. It can be used to treat algae, but it is not a cure. The root cause of algae is often CO2 deficiencies and ammonia in the water. Excel can be used to kill stubborn adult algae, but it is by no means a cure to the actual problem in the aquarium since it is not addressing the root cause of the algae in the first place.
Myth 5: Sunlight Causes Algae
Many people are often afraid to let any sunlight hit their aquariums since it "causes" algae. Light is only an issue in the aquarium when there are not enough CO2 and fertilizers to aid the plant growth. In a planted tank, think of light as the independent variable in a science experiment. This variable can be manipulated to be low light (half a watt of light per gallon) or extremely high light (direct sunlight like my outdoor tanks receive). CO2 and fertilizer demand are the dependent variables. As the light increases, CO2 and fertilizer demand increases as well. If you have a light as powerful as the sun hitting your tank and you don't have CO2 or fertilizers, then yes, you will get algae. However, if you have sufficient CO2 and fertilizers, let your tank bask in sunlight. Your plants will love it and display levels of coloration and texture that will reinvent the way you view your plants. CO2 levels and fertilizer levels can be adjusted to even handle the highest level of light.
Myth 6: CO2 injection is Not Natural
I often get inquiries regarding aquatic plants growing in the wild. They grow in crystal clear bodies of water with no visible algae. There is no visible CO2 injection into the water, so why is it necessary in our aquarium? It does seem unnatural to inject CO2 into the water, especially when aquatic plants in the wild supposedly grow submerged just fine without it. However, ponds, streams, and rivers contain a considerable amount of dissolved CO2. It's usually in the 20-30ppm range. This CO2 comes from tons of organic decay and CO2 enriched groundwater. The average aquarium only contains 2-3ppm of dissolved CO2, making CO2 injection in the planted aquarium quite logical in order to get the concentration up to 20-30ppm.
Links to CO2 enriched waters:
I hope that this guide is able to help some of you on your journey to an algae free planted tank. Patience and perseverance are key. Good luck!