How do I tackle this stinky algae?


A reader is frustrated by the unpleasant smell from his tank.

Q) My tank, which I’ve had for many years, is plagued with what I suspect is cyanobacteria. It’s relentless, covering the tank and everything in it in double-quick time. It’s also a very smelly algae, giving off an almost stagnant odour when I remove ornaments to clean them. 

I’ve tried regular and intensive cleaning, supplying new filter media, frequent water changes, special shop bought algae treatments, but it never completely goes. We’ve tried leaving the lights off for days which hasn’t worked. There are no fish in the tank — I lost them a while ago and I’m reluctant to buy any more while the algae is rife.

The lights are only on for 6-8 hours a day, and the tank is not in direct sunlight. We are considering stripping everything down and starting again. It’s soul destroying. Do you have any advice, please?


A) Neale replies: Cyanobacteria, also known as blue-green algae, are ubiquitous in aquatic environments, and their airborne and waterborne cysts will get into new aquaria within a few days, so it’s probably impossible to keep them out. While the use of antibiotics to kill cyanobacteria can work, it doesn’t stop them from coming back a few weeks later. Algae-killing potions may or may not have much useful effect, depending on the formulation, and it is important to remember that cyanobacteria are bacteria, not algae, and have a very different physiology.

With all of this said, it’s easy to imagine that cyanobacteria are unbeatable — but that simply isn’t true. Cyanobacteria are photosynthetic autotrophs, meaning they use light to make food. In this regard, they’re much like plants and algae. But their small size means their ability to compete with plants is very limited, and healthy plants will easily overgrow them, shading the cyanobacteria and starving them of light. This, in turn, means the cyanobacteria can’t make the food they need to respire and grow. While cyanobacteria will likely persist in such situations, they are after all just bacteria, and even in their thousands you’re never going to see them, much less care about them.

So, you need to make life difficult for them. Ensuring your true plants are thriving is the first step. A good benchmark is that you should be cropping your plants back every week or two. If you’re doing that, your plants will not only be outcompeting the cyanobacteria for light, but also removing mineral nutrients from the water more efficiently. The classic situation where you see annoying amounts of cyanobacteria is the tank that doesn’t have many fast-growing plants: perhaps a few Java Ferns and Anubias. 

Chances are the lighting is inadequate — unfortunately, the default condition for many basic fish tanks is to have inadequate lighting installed, and while sufficient for seeing your fish, there just isn’t enough light for fast-growing plants. What you want are species like Hygrophila polysperma and Vallisneria spiralis that grow incredibly quickly once settled in. If you have good lighting, and your plants aren’t growing quickly, check whether the substrate is right for them, and whether investing in carbon dioxide fertilisation is necessary for the species in question.

Bear in mind that floating plants can do the job as well as rooted plants, and are often easier to grow with more basic equipment, particularly floating Indian fern and Amazon frogbit. 

In my experience, outbreaks of cyanobacteria seem to be particularly linked to three things: direct sunlight, weak water currents, and high temperatures. In fact, in the wild they thrive in extreme conditions where algae and plants are lacking, including hypersaline pools and hot springs. In aquaria, tanks that receive periods of direct sunlight seem to get these hot, brightly-lit (if only for a short while) areas where the cyanobacteria get started. Weak water currents seem to exacerbate this, perhaps by not distributing the heat quickly, but it also seems to be the case that cyanobacteria develop best in still pockets of water, such as between pebbles or some other solid structure that affords them shelter. In strong water current the cyanobacteria wash away more easily, and this seems, to some degree, to keep them in check. 

The bottom line is that to avoid cyanobacteria, you need to create conditions they don’t like. Decent water currents along the substrate and enough bright light to promote rapid plant growth are the two best tools in your arsenal. As a reminder, true plants require 8-10 hours of good light a day, and if you reduce light duration to suppress algae or cyanobacteria, your plants will be even more adversely affected. Much better to increase light intensity, and have your higher plants outcompete both algae and cyanobacteria without any further intervention on your part. Plants are very good at keeping algae and cyanobacteria in check, having been doing so for tens of millions of years!