I've been slowly starting a multi-year campaign to get people to understand Cyanobacteria (BGA) better. There's a lot of misinformation in this hobby, but it's 2025, and times are changing. So let's give it a try! There will be a simple summary at the bottom.
Understanding Cyanobacteria (blue-green slime algae, BGA):
Cyanobacteria are a special "algae" in aquaria because they are actually
bacteria. These bacteria can utilize free N
2 nitrogen gas from the atmosphere to provide the necessary Nitrogen for growth
(source, source). Approx 78% of regular air is N
2. Like many other algae and plants, Cyanobacteria require some form of Nitrogen (N), Phosphorus(P), and Potassium (K) to grow and survive
(source). Without these things, they can't grow, same as plants.
This hobby has tons of misinformation and issues with correlation/causation. The main cause of BGA isn't too much light. It isn't too much unspecified "nutrients". It isn't (
technically) excess PO4. It isn't low flow, or high flow, or anything with oxygen (though they all might have a positive or negative effect in overall BGA growth).
In our freshwater systems, 95% of cyanobacteria outbreaks happen due to one main scenario:
How Nitrate(NO3)-Limited Environments cause BGA in aquaria:
Cyanobacteria
need N, P, and K to grow just like our aquatic plants. Not want, NEED, just like our plants.
However, our
aquatic plants have a preferred ratio of nearly ten NO3 to one PO4 to grow properly
(source). This means that NO3 is always going to be consumed faster than PO4 in our aquariums, so we need to provide ~ 10x NO3 for every 1 PO4, or our plants cannot grow (source). Liebig's Law of the Minimum and non-limiting environments for plant growth are always happening. This isn't opinion, this part is simple fact. You can't bake a cake if you don't have flour.
Often in our planted tanks, as plants increase their biomass, they need more and more NO3 every day. Besides Carbon (CO2), NO3 is the #2 consumed nutrient, so often aging tanks reach near-zero NO3 and "bottom-out". This limited environment could be due to aquasoil running out of NO3, or root tabs/liquid ferts/fish-waste not providing enough NO3 (and specifically, not providing enough NO3 to PO4 ratio).
Very likely your NO3-hungry aquarium plants aggressively consumed all of the NO3, leaving behind PO4. The plants can't use the PO4 because of Liebig's law: they NEED ~10x NO3 to consume ~1x PO4. If there is no NO3, your plants cannot consume the PO4 left behind.
But the cyanobacteria can consume the PO4 left behind.
"How can BGA consume leftover PO4 if there's a NO3-limited environment?" you ask... Because this algae can get it's Nitrogen needs from the FREAKING AIR (source, source).
In fact, 99% of the time someone asks for help with BGA, I ask them "What does your NO3 test kit show?"
Do you know what the answer has been, unfailingly, for years? ZERO NO3. ZERO. NO NO3. If they test for PO4, there is either a lot or a little, but very seldom none (BGA Can survive in PO4-limited environments, it just won't grow more -- only survive. Once established, it can survive for a long time before starving).
The BGA doesn't care if you have no NO3 left in your tank. It can get all the Nitrogen it wants from the atmosphere, dissolving into your tank's water, and there is nothing you can do to stop it from using the PO4 left behind to grow.
View attachment 8844
Source
- If the ratio favors Phosphorus (blue zone), either letting phosphate raise or nitrate decrease too much, we may easily have proliferation of blue-green algae (cyanobacteria).
- If the ratio favors Nitrogen (green zone), either letting nitrate raise or phosphate decrease too much, we may have proliferation of green algae species (usually GSA).
Why water changes (usually) don't help:
Since BGA can fulfill its Nitrogen needs from the atmosphere, it flourishes in NO3-limited environments where some PO4 is left unconsumed, while plants completely struggle or stop growing altogether.
When you perform a water change, you are removing
zero of the
zero NO3 left, but only whatever equivalent % of the PO4 left behind. This means that you are always leaving some PO4 in the aquarium if your NO3 is bottomed-out. If you have fish, perhaps the fish waste they produce from the flakes they eat "fertilizes" with some NO3 and PO4... but the 10x NO3-consuming plants quickly bring the NO3 back down to zero, leaving behind PO4 again in the aquarium, and providing the exact environment for BGA to thrive.
So beginners do another water change, and another, and to their surprise, the BGA not only stays, but grows more.
"What the heck! I was told water changes help with algae". That's true if you understand
how, but in the case of BGA it often makes the scenario worse by potentially removing the very thing your plants are starving for most (NO3).
If water changes alone don't help, how do I treat BGA? It's killing my plants, or it's on my substrate!
Because it's a bacteria, BGA can (and should) be initially treated with an aquarium-specific antibiotic. Nearly all species of cyanobacteria are gram-negative bacteria
(source). In our aquarium, the "good" bacteria that are part of the nitrogen-cycling bacteria in our filters and substrate are gram-positive bacteria (source). This means that you can safely treat cyanobactera with gram-negative antibiotics without affecting your tank's main bacterial health. I have done this many, many times and the only effect I noticed was some potential melting of Vals with a 4x strength dose. However, in 90% of my cases all plants, shrimp, fish, and filter bacteria were fine while the cyanobacteria disappeared for a few days.
Some will suggest a blackout to treat it, which might work -- BGA do need light to survive and to grow, but so do your plants. Blackouts are pretty outdated treatment methods for algae issues. We know better now: Healthy plants prevent algae, and a blackout hurts plants the same as it hurts BGA. You might kill the BGA with a blackout only to keep getting algae issues down the road.
Increasing oxygen has also been reported to help some, but I got BGA in my extremely surface-agitated 150p aquarium with TONS of dissolved oxygen. Remember, these might have some effect on BGA, but are not the main cause and shouldn't be considered the main treatment.
How to treat BGA in the short term:
- You need to first use a hose/siphon to remove as much BGA as possible. If infected leaves are heavy with BGA, trim them. They will usually not recover well, and trimming will encourage new BGA-free growth.
- After manual removal, treat with a trusted gram-negative antibiotic. I have tried multiple:
View attachment 8845
Blue Life Green Cyano Rx works, but I prefer:
View attachment 8846
Ultralife Blue-Green Slime Remover. I have used this product to great success over the years on countless tanks I let bottom-out of NO3 with the presence of PO4.
How do I keep BGA from coming back again and again and again?
The simple answer is: stop creating NO3-limited environments.
Manually remove the BGA first, then dose with a recommended BGA antibiotic. Don't let your aquarium bottom-out of NO3, or Increase your NO3 dose and slighly decrease your PO4 dose. Don't do big water changes
without re-adding the NO3 your plants need, or cyanobacteria will consume the PO4 left behind. Water changes are supposed to help balance nutrients, not "remove" them. Excess nutrients have never really caused algae -- specifically, nutrients in excess in the presence of one or more nutrient-limited environments cause algae (as well as dead/rotting plants and poor waste management).
Understanding how to create a non-limited environment for ALL nutrients our plants need (C,N,K,P,Ca,Mg,SO4,Fe,Traces in that order) is the key to success. You can do this through nutrient-rich aquasoil, root tabs, fish waste, or liquid fertilizers -- but understand that anytime you create imbalances of nutrients through limited environments,
you will get algae.
If you have
PO4 present but absolutely zero NO3, you get
cyanobacteria (BGA) and/or Green Hair Algae (GHA) (depending on CO2 concentration and K concentration).
If you have
plenty of NO3 but absolutely bottomed-out PO4, you get
Green Spot Algae (GSA).
If you have fluctuating CO2 levels and lots of complex waste organics from fish waste, rotting plants, or a generally unclean tank, you get Black Beard Algae (BBA).
Healthy plants do not get algae. In an aquarium where there is essentially greater-than-zero of
"C,N,K,P,Ca,Mg,SO4,Fe,Traces" at all times (nothing bottomed-out),
your plants will never be starved for anything and will almost always outcompete/prevent algae growth.
You can do this through aquasoil, root tabs, fish waste, or liquid/dry fertilizers. Learning to test/observe and understand these scenarios will help you diagnose what's
actually going on in your aquarium. BGA is no different -- when your NO3 hits near zero, plants can't consume PO4, and BGA uses the N from the atmosphere to grow in your aquarium.
It's not about specific amounts or even specific ratios -- as long as your NO3 and PO4 are anything BUT zero, and they are somewhere between 5:1 and 20:1, you will not get cyanobacteria, period.
Summary of Cyanobacteria and how to treat it:
- BGA is a photosynthetic bacteria that has the ability to get the nitrogen it needs from either the atmosphere or our aquarium.
- Plants cannot consume 1 PO4 if there isn't ~10x NO3 available.
- When plants can't consume PO4 due to NO3 limitation, BGA can take N2 from the atmosphere and consume the PO4.
- Water changes don't help if they are only making the NO3 limitation worse
- To treat, remove as much manually as you can. Trim heavily affected leaves if necessary.
- While not required, using an aquarium-safe gram-negative antibiotic like UltraLife Blue Green Slime remover will melt most BGA.
- Once removed/treated, prevent recurring outbreaks by either increasing your NO3 concentration in your tank, or reducing your PO4.
In most cases, your plants are likely starving for NO3, so that's the easiest once to increase -- your plants will thank you!
Hydrogen peroxide alone has never been enough for me.
