Algae is the most frustrating problem in the planted aquarium hobby — and also the most misunderstood. The instinctive reaction when it appears is to cut fertiliser and reduce light. In most cases, both of those responses make the situation significantly worse. Here’s what’s actually driving your algae, how to identify the specific type you’re dealing with, and what to do about it.

The real cause of algae: imbalance, not abundance

Algae doesn’t appear because you have too many nutrients in the water. It appears because your plants aren’t consuming nutrients fast enough — leaving a surplus that algae can exploit. This distinction matters enormously because it completely changes how you should respond.

Healthy, actively growing plants are your most powerful defence against algae. They compete with algae for the same nutrients and light, and when they’re growing vigorously they win that competition decisively. Algae cannot establish itself in a tank where healthy plants are rapidly consuming all available nutrients.

The problem occurs when plants are struggling — because of inconsistent fertiliser dosing, specific nutrient deficiencies, poor or unstable light, or fluctuating CO₂. When plants slow down or become stressed, they consume nutrients slowly. The surplus builds up in the water, and algae fills the gap. The plants aren’t the problem — the conditions weakening the plants are the problem.

Cause 1: Inconsistent dosing

This is the most common driver of algae in planted tanks — and it’s the easiest to fix once you understand it. When you dose every other day, forget for a few days and then double-dose to compensate, or switch between different products irregularly, you create boom-and-bust nutrient cycles in the water.

Plants can’t store nutrients for more than a day or two. They consume what’s available in the water right now, as photosynthesis demands it. A day without nutrients is a day of stress. Two or three days without nutrients and growth stalls noticeably. During that gap, any nutrients already dissolved in the water from fish waste or substrate leaching remain available — and algae, which is opportunistic, will exploit them immediately.

Daily micro-dosing solves this problem at the root. Small, consistent additions every day keep nutrient levels at a stable, low baseline that plants consume as fast as you add them. There’s never a meaningful surplus for algae to exploit between doses, and there’s never a period of plant stress from nutrient starvation. The plants stay ahead of the algae permanently.

For the full dosing guide including dose amounts by tank size and light level, see: How Much Aquarium Fertiliser Should I Use?

Cause 2: Fluctuating CO₂

If you’re injecting CO₂ and still experiencing persistent algae despite consistent fertilisation, fluctuating CO₂ levels are the most likely culprit — more so than anything to do with nutrients. CO₂ is the primary raw material for photosynthesis. When it’s unstable — spiking when the cylinder pressure is high, dropping as the cylinder empties, or being unevenly distributed around the tank — plants can’t photosynthesise efficiently regardless of how well you’re dosing fertiliser.

When CO₂ drops, photosynthesis slows, plants consume fewer nutrients, and the surplus left in the water immediately becomes available to algae. Then when CO₂ rises again, plants can’t instantly return to full growth rate — there’s a lag. During that lag, algae takes hold.

Black beard algae (BBA) in particular is almost always a CO₂ stability issue. It tends to colonise areas of poor flow first — spots where CO₂ delivery is inconsistent — and spreads from there. If you’re seeing BBA specifically, check your CO₂ before changing anything else.

Cause 3: Too much light for the plant mass present

New tanks are particularly vulnerable to algae because the plants haven’t yet established themselves. Root systems are still developing, the plants are acclimatising from their emersed or tissue culture growth forms, and nutrient uptake is slow. Running your lights at full intensity for 10 hours a day in a brand new tank provides far more photosynthetic energy than plants can currently use — and algae exploits that surplus energy immediately.

In an established tank, hair algae, green thread algae, and green spot algae on the glass can all indicate excessive light relative to the current plant mass and nutrient uptake rate. The relationship is triangular: light drives demand for nutrients and CO₂. Too much light relative to nutrients and CO₂ creates an algae opportunity.

This is also why reducing light is sometimes the correct first response to algae — not because light causes algae directly, but because reducing light reduces the energy available to algae while plants catch up. It’s a temporary measure, not a long-term solution.

Cause 4: Specific nutrient imbalance

Not all algae problems relate to overall nutrient level — some are driven by specific imbalances between elements. Nitrogen deficiency weakens plants so they can’t compete effectively. Low phosphate, counterintuitively, is associated with green spot algae on the glass — the algae exploits the gap left by phosphate-starved plants. Very high phosphate relative to nitrogen creates conditions some algae types prefer.

This is where individual compounds become genuinely valuable in diagnosis and treatment. With a pre-mixed all-in-one liquid fertiliser, you can’t increase nitrogen without increasing everything else, and you can’t reduce phosphate without reducing everything else. With individual compounds, you can address exactly what’s imbalanced. For the full guide to identifying nutrient deficiency patterns: Why Are My Aquarium Plant Leaves Turning Yellow?

For example: if you’re seeing green spot algae on the glass — hard, circular green dots that don’t wipe off easily — try increasing your KH₂PO₄ dose slightly. If you’re seeing cyanobacteria (blue-green slime, often appearing on substrate or low-flow areas), increase your KNO₃ dose and check your tank circulation.

Reading your algae — what each type tells you

Different algae types are associated with different underlying problems. Identifying what you’re actually dealing with is half the battle — and it tells you specifically what to address first.

If you have shrimp in an algae-affected tank

When algae appears, some hobbyists consider copper-based algaecides or treatments. If you have shrimp, snails, or any invertebrates in the tank — do not do this under any circumstances. Copper is lethal to shrimp even at very low concentrations, and many copper-based products will destroy an entire shrimp colony within hours. There is no margin for error here.

The correct approach for shrimp tanks with algae is manual removal combined with identifying and fixing the underlying cause. A healthy shrimp population actually helps with algae management over time — shrimp graze continuously on many algae types including biofilm, green dust algae, hair algae, and diatoms. Getting the tank conditions right so the plants thrive is the best thing you can do for both the plants and the shrimp.

For the complete guide to safely fertilising tanks with shrimp: Is Aquarium Fertiliser Safe for Shrimp?

Recovering from an established algae outbreak

If you’re already dealing with significant algae rather than just the first signs of it appearing, here’s the recovery sequence that works:

Get your dosing right