Algae 101: the good, the bad, and the ugly

Why algae exists in your tank

Algae is a photosynthetic organism that needs two things to grow: light and nutrients. Your aquarium has both in abundance. Light comes from your fixture (and potentially a nearby window); nutrients come from fish waste, uneaten food, decomposing plant matter, and your tap water. The question isn't whether algae will appear - it will - but whether the conditions in your tank favour a nuisance bloom or just the background level that every healthy aquarium carries.

A thin green film on the back glass, a faint coating on a piece of driftwood, algae on rocks that your nerites are grazing - none of that needs fixing. It signals that your tank is biologically active. The problem starts when algae colonises your plants, clouds the water, or forms thick mats you didn't notice until they'd taken over half the hardscape. That's not a livestock problem or a filtration problem in isolation; it's almost always a lighting or nutrient problem first.

The five types you'll actually encounter

Misidentifying algae is the most common reason treatments fail. Here's what each type looks like and what it's telling you about your tank.

Green water (suspended algae)

Your tank water turns pea-soup green and you can't see the back glass. This is caused by single-celled algae suspended in the water column, not growing on surfaces. It almost always follows a combination of excess light and elevated nutrients - particularly if your tank gets direct sunlight or you've recently overfed. Green water is harmless to fish but visibility drops to near zero and it looks terrible. A UV steriliser clears it fast (usually 3-5 days); a 3-day blackout works too but needs to be total - not a single photon.

Hair and thread algae

Long, fine strands that tangle around plant leaves, attach to filter intakes, and form mats on the substrate. It ranges from bright green to yellowish depending on how long it's been there. The cause is almost always too much light combined with a nutrient imbalance - often high nitrate, sometimes elevated phosphate. Amano shrimp are genuinely excellent at controlling this type. Manual removal by twirling strands around a toothbrush speeds things up considerably.

Black beard algae (BBA)

Audouinella sp. - small, dense tufts of dark grey or black fuzz that attach to the edges of slow-growing plant leaves, anubias, hardscape, and the rims of equipment. It feels slightly rough to the touch. BBA is one of the most stubborn algae types in the hobby and it's strongly associated with fluctuating CO₂ levels, though high organic load matters too. If you're running pressurised CO₂ and still getting BBA, look at your bubble count consistency and whether your diffuser is running the same hours as your lights.

Brown algae (diatoms)

A dusty, light-brown coating on glass, substrate, and decor. It wipes off easily with a finger. Diatoms are the classic new-tank algae - they dominate in the first few weeks because silicates in tap water and low-competition conditions favour them. In most tanks they fade on their own within 6-8 weeks as the biological system matures. Otocinclus catfish graze them enthusiastically. If diatoms persist in an established tank, check your silicate levels and consider a silicate-removing resin in your filter.

Cyanobacteria (blue-green "algae")

Despite the common name, this isn't algae - it's a photosynthetic bacteria. It forms slimy, sheet-like mats in blue-green, dark green, or reddish-purple. It smells unpleasant. It grows over everything: substrate, plants, rocks, equipment. Cyano thrives in low-flow areas with high nutrients and often appears when nitrate is low but organic waste is high (which confuses people who test nitrate and see "good" numbers). Improving flow, doing large water changes, vacuuming the substrate thoroughly, and a 3-day blackout usually set it back. Persistent cases sometimes need an antibiotic treatment (erythromycin), though this should be a last resort because it affects your beneficial bacteria too - re-cycling your tank afterward is worth planning for.

What actually causes algae outbreaks

The hobby sometimes treats algae as mysterious, but the causes are consistently the same. Light duration and intensity, nutrient levels (nitrate, phosphate, organic waste), and CO₂ stability explain nearly every case. Get those three variables right and most algae stays manageable without any products.

Light: duration matters more than intensity

Most planted tanks run fine on 6-8 hours per day. Ten hours is too long for most setups. Direct sunlight through a window is a guaranteed algae trigger - it adds uncontrolled, high-intensity light that overwhelms any photoperiod discipline you've built into your schedule. Use a timer, run 6-8 hours, and put something between your tank and any south- or west-facing window.

Nutrients: overfeeding is the biggest culprit

Every uneaten flake that sinks to the substrate breaks down into ammonia, then nitrate and phosphate. These are direct algae fertilisers. Feed only what your fish consume in 2-3 minutes, once or twice a day, and gravel-vac mulm from the substrate during water changes. Nitrate above 40 ppm consistently, or any detectable phosphate, means your export isn't keeping up with your input. Regular water changes are the most reliable export mechanism - see our guide on how often to do water changes if you're unsure on frequency.

CO₂ fluctuation and planted tanks

This applies specifically to injected CO₂ setups. BBA in particular is strongly linked to CO₂ inconsistency - running CO₂ on a different schedule to your lights, letting the bubble count drift, or having surface agitation that outgasses CO₂ before plants can use it. If you're injecting CO₂, run it on the same timer as your lights (start 1 hour before lights on, stop 1 hour before lights off) and aim for stable dissolved CO₂ around 20-30 ppm, indicated by a drop checker showing lime green.

Treatments that actually work

Manual removal first

Before anything else, physically remove as much algae as you can. Scrape the glass with a razor blade or magnetic cleaner, pull hair algae out by hand or with a toothbrush, prune heavily affected plant leaves rather than trying to clean them. This removes the biomass and the nutrients locked in it. Always follow manual removal with a water change to export the dislodged material.

Reduce light and nutrients in parallel

Cut your photoperiod to 6 hours temporarily during an outbreak. Cut feeding to once daily, small amounts. Do 30-40% water changes twice weekly until parameters stabilise. Vacuum the substrate every water change. This combination resolves most common algae outbreaks within 2-4 weeks without any additives.

Cleanup crew: what they can and can't do

The right crew makes a real difference to ongoing algae management. Amano shrimp (Caridina multidentata) are the most effective algae grazers in freshwater - a group of 6-10 in a medium tank makes a visible difference to hair algae and soft green algae. Nerite snails are exceptional at glass and hard surface algae and won't reproduce in freshwater. Otocinclus catfish graze diatoms and green film from leaves without touching plants. Siamese algae eaters (true Crossocheilus oblongus, not the lookalike flying fox) will eat BBA when young, though they often get lazier with age. None of these animals fix a root-cause problem. Overstocked, overfed, or over-lit tanks will outproduce any cleanup crew.

Targeted treatments for stubborn cases

BBA: spot-treat with liquid carbon (glutaraldehyde-based products like Seachem Excel or similar). Turn off your filter, dose the affected area directly with a syringe, wait 5-10 minutes before restarting flow. BBA turns pink-red within a day or two and then gets grazed by shrimp or dies off. Multiple treatments over 2-3 weeks are usually needed. Raising CO₂ consistency is the long-term fix.

Green water: a UV steriliser is the fastest reliable solution. Alternatively, a 3-day total blackout (cover the tank completely) starves the suspended algae of light. Your fish and established plants will be fine for 3 days in the dark; algae won't. After the blackout, address why it happened - usually a light/nutrient excess or a brief direct-sunlight exposure.

Cyanobacteria: improve flow with a powerhead or wavemaker aimed at low-flow areas, do large water changes, vacuum substrate thoroughly, and try a 3-day blackout. If it returns persistently, hydrogen peroxide spot-treatment (3% solution, 1-2 ml per 10L directly applied with filter off) is effective and breaks down into water and oxygen quickly. Erythromycin works but nukes your bacterial colony - treat it as a last resort and be prepared to re-cycle.

What most guides miss

The hobby focuses heavily on adding things to fix algae - products, animals, additives. Less attention goes to the boring fundamentals: consistent light schedules, not overfeeding, and regular maintenance. Algae outbreaks in established tanks almost always trace back to a change in routine - a holiday where someone fed too much, a light timer that got reset, a filter that got skipped during a house move. Keeping logs of your parameters and light schedule makes these changes visible. That's exactly what App-aquatic is built for - tracking test results over time so you catch a rising nitrate trend before it becomes a bloom.

One thing genuinely debated in the hobby: the role of phosphate. Some experienced planted tank keepers deliberately dose phosphate as a plant fertiliser and control algae through strong plant competition and CO₂. Others keep phosphate at zero and see clean tanks. Both approaches work in the right context. What doesn't work is zero phosphate, weak plants, insufficient CO₂, and a 10-hour photoperiod - that combination reliably produces algae regardless of anything else you do.