Does aquarium algae spread? The science of how it takes over

Is algae “contagious”?

It is one of the most common observations in the planted-tank hobby, and it feels sinister: you notice a single tuft of black beard algae (BBA) on a piece of driftwood, you leave it — and over the following weeks it appears on the edges of your plant leaves, on the filter outlet, on the heater. It looks for all the world as though the algae is spreading, jumping from surface to surface like an infection. So the natural question is: does having algae in your tank mean it will inevitably take over? Is it contagious — or is that not supposed to happen?

The honest answer is a satisfying “yes, but not the way it looks.” There is a real, well-documented mechanism by which an established patch of algae seeds the rest of the tank. But algae is not a disease that infects healthy tissue. It is an opportunist, and whether it establishes on a new surface is decided far more by the conditions at that surface than by how close it sits to the parent patch. Understanding the difference is the key to actually stopping the spread — and to knowing when it is a normal nuisance versus a warning sign.

Yes — algae genuinely spreads, and here is how

Algae are not passive smears that sit where they land. They reproduce, and they disperse, through three routes that any aquarist can picture:

• Spores. Microscopic reproductive cells released into the water, carried on the current, that settle on a surface and germinate into a brand-new individual.
• Fragmentation. A piece of an existing growth breaks off — snapped by flow, a fish, or your tweezers — drifts away, re-attaches, and carries on growing.
• Vegetative growth. The existing colony simply enlarges at its margins.

For black beard algae, the first route is the important one. Aquarium “BBA” is not a plant and is not even a green alga: DNA barcoding has shown that the vast majority of it is a freshwater red alga, Compsopogon coeruleus, spread around the world largely by the aquarium trade itself.² Its biology is the crux of the whole question: Compsopogon reproduces predominantly by asexual monospores — single, water-dispersed spores that germinate directly into a new clone without any fertilisation.¹ In other words, an established tuft acts as an ongoing spore source: while it sits there it keeps shedding propagules into the water column — how fast varies with its condition rather than being perfectly constant — and the flow carries whatever it sheds around the tank.

But spores are almost everywhere — so why isn’t every tank drowning?

Here is the part that reframes the whole problem. If algal spores were the limiting factor, every aquarium would be a green disaster, because algal propagules are effectively ubiquitous. They arrive on new plants, on the water you add, on nets and equipment moved between tanks, and they persist in almost every established system. The presence of spores is not what decides whether you get an algae outbreak — because in virtually any established tank, the spores are already there.

What actually decides it is whether those spores can establish and grow once they land. And the ecology here is well studied: the success of a spreading organism is co-regulated by two things — propagule pressure (how many spores arrive) and biotic resistance (how effectively the existing community resists them).⁴ (That framework comes from broader invasion ecology — much of the direct evidence is marine — rather than from controlled aquarium studies, but the logic transfers directly.) In an aquarium, your biotic resistance is your plants. A tank full of healthy, fast-growing, well-fed plants is hostile territory for a settling spore: the plants are drawing down the very nutrients and carbon the algae needs, and healthy, actively growing tissue is a far poorer surface to colonise than weak or damaged tissue.

So the spread you are seeing is not contagion in the medical sense. Your plants did not “give” the algae to one another. Rather, the water is full of propagules from the parent patch, and they are catching on precisely the surfaces where the local conditions have tipped in the algae’s favour.

The pattern of the spread tells you what is wrong

Watch where BBA appears and it stops looking random. It has strong preferences, and every one of them is a clue:

• High-flow zones. BBA loves current. It colonises filter outlets, spray bars, CO₂ diffusers and the leading edges of leaves that sit in the flow, because a steady current delivers a constant supply of dissolved carbon and nutrients straight to it. This is why it so often first appears on leaf margins — the parts that flap in the current. (More on getting flow right in the aquarium flow guide.)
• Old, slow and stressed tissue. A settling spore tends to establish more readily on an ageing leaf, a slow-growing plant, or tissue already weakened by poor conditions than on vigorous new growth — a pattern every experienced grower will recognise, though it rests more on practitioner observation than on laboratory measurement. Algae is the classic coloniser of surfaces that are not defending themselves.
• Unstable carbon and organic waste. In injected tanks, fluctuating CO₂ is the most-cited trigger: when carbon supply swings, plants are forced to keep re-adjusting and leak more dissolved organic compounds, which appear to give algae the opening it needs. Decaying leaves and detritus add to that organic load. The evidence for the CO₂-fluctuation mechanism is largely observational and physiological rather than from controlled trials — the full case, and what is still hypothesis, is laid out in our black beard algae article and the CO₂ stability guide.

Notice that none of these is “the algae was nearby.” Proximity to the parent patch raises the spore count, but it is the flow map and the health of each surface that determine where those spores actually take hold.

So — is the spreading “supposed” to happen?

It depends entirely on what it is spreading onto:

• Spreading onto hardscape, equipment, old leaves and high-flow spots is normal and expected. These are exactly the surfaces a spore-shedding red alga will exploit, and their colonisation tells you the algae has a source and a supply of suitable, undefended real estate — not that anything exotic is wrong.
• Spreading onto the healthy new growth of vigorous plants is a genuine red flag. When BBA starts colonising fresh, fast-growing tissue, it means biotic resistance has collapsed — the plants are no longer growing well enough to hold their own surfaces, usually because of the same underlying condition (unstable carbon, low light-to-nutrient balance, or a heavy organic load) that is feeding the algae in the first place.

Put simply: a little BBA creeping onto driftwood in the current is the tank telling you there is a spore source and some spare surfaces. BBA marching across your healthiest plants is the tank telling you the plants are losing.

Two levers, and you need both

Because the spread is driven by propagule pressure and conditions together, tackling only one of them fails — which is exactly why so many aquarists feel they “can’t get rid of it.”

Do only the first and the algae returns, because the conditions keep re-seeding from the ubiquitous spore pool. Do only the second and progress is painfully slow, because your existing patches keep firing fresh spores into the water the whole time. Done together, they work: you remove the sources while simultaneously making every surface harder to colonise, and the balance tips back toward the plants.

What is established, and what is still debated

In keeping with our editorial standard, it is worth being clear about how solid each part of this is:

• Well established. That aquarium BBA is Compsopogon coeruleus; that it reproduces and disperses mainly by water-borne monospores and fragments; and that the establishment of any spreading organism is governed by propagule pressure together with biotic resistance. These are supported by peer-reviewed phycology and invasion ecology.^1,2,3,4
• Practitioner consensus, thinner direct evidence. The specific link between fluctuating CO₂ and BBA is well-supported by plant physiology and by the near-universal experience of experienced high-tech growers, but has limited controlled experimental confirmation. Treat it as a strong working hypothesis, not a proven law.
• Frequently overstated. The idea that plants chemically suppress algae through allelopathy is real for a handful of species (hornwort is the best-documented) but is routinely exaggerated across the hobby. And “plants outcompete algae” is true chiefly through nutrient and carbon competition — at genuinely low nutrient levels the relationship can even reverse, which is why simply starving a tank rarely fixes algae without also fixing plant health.

The takeaway is reassuring, though: algae spreading is not a mysterious contagion, and a tank with a little algae is not doomed. It is a source shedding spores into water that already carries plenty of them, taking hold wherever your conditions have left the door open. Close those doors — healthy plants, stable carbon, low waste, sensible flow — and remove the visible sources, and the “spread” stops having anywhere to go.

1. Necchi, O. & Dip, M.R. Reproductive phenology and ecology of the freshwater red alga Compsopogon coeruleus: predominantly asexual reproduction by monospores. Reproductive biology of Compsopogon (Compsopogonales, Rhodophyta).
2. Sherwood, A.R. et al. (2020). Hidden introductions of freshwater red algae via the aquarium trade exposed by DNA barcodes. bioRxiv, 2020.06.30.180042.
3. Simberloff, D. (2009). The role of propagule pressure in biological invasions. Annual Review of Ecology, Evolution, and Systematics, 40, 81–102.
4. Xu, C.-Y. et al. (2018). Biotic resistance and vegetative propagule pressure co-regulate the invasion success of a marine clonal macrophyte. Ecology and Evolution, 8(21), 10231–10241 (PMC6226502).