Algae blooms: 7 causes and matching fixes

The algae bloom is one of the most common and most frustrating problems in the aquarium and pond hobby, and the algae bloom is one of the most commonly misdiagnosed. The owner who sees green water, brown water, or green hair on the rocks assumes that the algae is the problem, and the owner treats the algae. The owner is wrong. The algae is not the problem. The algae is the symptom. The problem is the underlying condition that is allowing the algae to bloom, and the treatment that does not address the underlying condition is the treatment that does not work. The algae comes back. The algae comes back stronger. The owner adds more treatment. The cycle continues, and the tank or the pond is in a chronic green state.

This article is going to walk you through the seven most common causes of algae blooms in freshwater and saltwater systems, and the specific matching fix for each one. The framework that I describe below is the framework that I use in my own system management and in my consultations, and the framework is the one that produces the durable fix rather than the recurring symptom. I am a pond and aquarium keeper with 20+ years of experience, and the algae bloom is one of the most common topics of my consultations. The cases that go well share a common feature: the owner has identified the underlying cause and has addressed the cause. The cases that go poorly share a different common feature: the owner has treated the symptom and has not addressed the cause.

I want to be clear at the start: the algae is not the enemy. The algae is a normal part of the aquatic ecosystem, and the small amount of algae is a sign of a healthy system. The algae becomes a problem when the algae blooms beyond the normal range, and the bloom is a signal that something in the system is out of balance. The fix is to restore the balance, and the balance restoration is the durable fix.

Cause 1: Excess light

The most common cause of algae blooms, in both freshwater and saltwater systems. The algae, like all photosynthetic organisms, requires light to grow, and the algae grows in proportion to the light it receives. The aquarium or pond that is exposed to excessive light (intensity, duration, or both) is a system that is providing the algae with the energy to bloom.

The signal pattern. The green water (a free-floating algae bloom that turns the water green) or the green hair (a hair algae bloom that covers the rocks, the plants, and the glass) develops after a change in the lighting (a new light, a longer photoperiod, a brighter light) or in a system where the lighting has always been excessive.

The fix. Reduce the lighting. Specifically:

• Reduce the photoperiod to 6–8 hours per day. Most planted tanks and most reefs do well with 6–8 hours of light. The tank that is receiving 10–12 hours of light is the tank that is providing the algae with an excess of energy.

• Reduce the intensity if the light is adjustable. The modern LED lights are adjustable, and the intensity should be reduced to the minimum that supports the plants or the corals. The plants and the corals adapt to the lower intensity over 1–2 weeks.

• Add a UV sterilizer. The UV sterilizer kills the free-floating algae as the water passes through the sterilizer, and the UV sterilizer is a mechanical fix for the green water specifically. The UV sterilizer does not address the underlying cause, but the UV sterilizer provides an immediate visual improvement.

• Provide a "siesta" period. Some reef keepers run their lights with a midday siesta (2–3 hours off in the middle of the day), which reduces the total photoperiod and reduces the algae growth. The siesta also has the effect of mimicking the natural light cycle of the reef (where there is a brief period of reduced light in the middle of the day).

Cause 2: Excess nutrients (nitrate and phosphate)

The second most common cause. The algae, like all plants, requires nitrogen and phosphorus to grow, and the algae grows in proportion to the available nutrients. The aquarium or pond that has an excess of nitrate, an excess of phosphate, or both, is a system that is providing the algae with the nutrients to bloom.

The signal pattern. The algae bloom is associated with a measurable increase in the nitrate (above 20 ppm in freshwater, above 10 ppm in saltwater) or the phosphate (above 0.5 ppm in freshwater, above 0.05 ppm in saltwater). The bloom may be triggered by overfeeding, by an under-cycled tank, by infrequent water changes, by the use of nutrient-rich tap water, or by a recent die-off of plants or animals.

The fix. Reduce the nutrients. Specifically:

• Reduce the feeding. The overfed tank is the tank that is producing the excess nutrients. The feeding should be reduced to the amount the animals consume in 2–3 minutes, once or twice per day, and the uneaten food should be removed.

• Increase the water changes. The water change is the simplest and most effective way to reduce the nitrate and the phosphate. The water change should be 25–50% per week for a moderately stocked tank, and 50% per week for a heavily stocked or heavily fed tank.

• Add macroalgae to the refugium (for saltwater systems). The macroalgae (Chaetomorpha, Caulerpa, Gracilaria) consumes the nitrate and the phosphate as nutrients, and the macroalgae is one of the most effective natural filters for a saltwater system.

• Use a phosphate-removing media. For systems with persistently high phosphate (above 0.1 ppm in freshwater, above 0.05 ppm in saltwater), a phosphate-removing media (GFO, lanthanum-based media) can be used in the filter to remove the phosphate directly.

• Verify the source water. The tap water in some regions is high in nitrate (from agricultural runoff) or high in phosphate (from water treatment processes). The source water should be tested, and the use of RO/DI water should be considered if the source water is the source of the nutrients.

Cause 3: Excess organic waste (the "dirty tank" cause)

The third most common cause. The aquarium or pond that has an accumulation of organic waste (uneaten food, fish waste, plant debris, dead algae) is a system that is producing a constant supply of nutrients as the organic waste decomposes. The algae blooms in the nutrient-rich environment produced by the decomposing waste.

The signal pattern. The algae bloom is associated with visible debris in the tank (uneaten food, fish waste, plant debris), with a substrate that is dark and anaerobic (the substrate is "dirty"), with a filter that is overdue for cleaning, or with a tank that has not had a deep clean in months.

The fix. Clean the tank. Specifically:

• Vacuum the substrate. A gravel vacuum or a sand siphon is used to remove the accumulated organic waste from the substrate. The vacuuming should be done as part of the weekly water change, and the vacuuming should remove the visible debris and the trapped waste.

• Clean the filter. The filter is the home of the beneficial bacteria, and the filter should not be cleaned with chlorinated tap water (the chlorine kills the bacteria). The filter should be cleaned in the tank water that has been removed during the water change, and the filter media should be gently rinsed to remove the trapped debris.

• Remove the dead plant material. The dead or dying plant material is a source of nutrients as it decomposes, and the dead material should be removed promptly.

• Reduce the bioload if necessary. The tank that is overstocked (too many fish for the tank size) is a tank that is producing more waste than the system can handle, and the bioload should be reduced if the tank is overstocked.

Cause 4: Inadequate water circulation

The fourth most common cause. The aquarium or pond that has dead spots (areas with little or no water movement) is a system that is allowing the nutrients to accumulate in the dead spots, and the algae is growing in the nutrient-rich dead spots. The inadequate circulation is particularly common in large tanks with a single return pump, in tanks with heavy décor that blocks the flow, and in ponds with insufficient pump sizing.

The signal pattern. The algae is concentrated in specific areas of the tank (the corners, the back wall, the areas behind the décor), and the algae is not growing in the areas with good water movement. The dead spots can be identified by observing the water movement (the food or the debris that should be moving with the flow is not moving in the dead spots).

The fix. Improve the circulation. Specifically:

• Add a powerhead or a circulation pump. The powerhead or the circulation pump is added to the tank to improve the water movement in the dead spots, and the powerhead is positioned to push the water through the dead spots.

• Reposition the return pump outlet. The return pump outlet may be positioned in a way that creates dead spots (the flow is concentrated in one area, and the other areas are dead spots). The repositioning of the outlet (or the addition of a second outlet) can improve the circulation.

• Rearrange the décor. The décor that is blocking the flow should be rearranged or removed. The caves, the rock structures, the driftwood that block the flow are creating dead spots, and the dead spots are the algae hotspots.

• Verify the flow rate. The tank should have a flow rate of 5–10x the tank volume per hour (for freshwater) or 10–20x (for saltwater, especially reef). The flow rate that is below the appropriate range is a flow rate that is producing dead spots.

Cause 5: Inadequate macroalgae or plant competition

The fifth cause, specific to the planted tank or the refugium. The planted tank that has too few plants (or that has plants that are not growing well) is a tank that is not consuming the nutrients that the algae needs, and the algae blooms in the absence of competition. The refugium that has too little macroalgae is a refugium that is not consuming the nutrients, and the nutrients accumulate in the display tank.

The signal pattern. The algae bloom is associated with a sparse or unhealthy plant growth, with a refugium that has been recently harvested (the macroalgae has been removed and has not regrown), or with a planted tank that is in the early establishment phase (the plants have not yet grown enough to consume the nutrients).

The fix. Increase the plant or macroalgae mass. Specifically:

• Add more plants to the planted tank. The fast-growing stem plants (Hygrophila, Rotala, Ludwigia, Bacopa) are the most effective competitors, and the addition of these plants can rapidly reduce the available nutrients.

• Increase the macroalgae in the refugium. The macroalgae should be a substantial portion of the refugium (the macroalgae should fill at least 50% of the refugium volume), and the macroalgae should be harvested regularly to promote new growth.

• Address the plant health issues. The plants that are not growing well are the plants that are not consuming the nutrients, and the plant health issues (the inadequate lighting, the CO2 deficiency, the nutrient deficiency) should be addressed.

Cause 6: New tank syndrome (the unestablished tank)

The sixth cause, specific to the new tank. The new tank is a tank that has not yet developed the stable bacterial population, the stable plant mass, and the stable microfauna that compete with the algae. The new tank is, in many ways, an algae paradise — the nutrients are available, the competition is minimal, and the algae blooms.

The signal pattern. The algae bloom develops in the first 2–8 weeks of a new tank, before the tank is fully cycled and before the plant mass is fully established. The bloom is often the "brown algae" (diatom) bloom that is characteristic of the new tank, or the "green hair" bloom that develops if the nutrients are high.

The fix. Wait and manage. Specifically:

• The diatom bloom resolves on its own. The brown diatom algae that develops in the new tank is a normal part of the cycling process, and the diatom bloom resolves on its own within 2–6 weeks as the bacterial population matures and the diatom's silica supply is consumed.

• Add a cleanup crew. The cleanup crew (amano shrimp, nerite snails, otocinclus catfish for freshwater; hermit crabs, snails, sea urchins for saltwater) consumes the algae and helps to control the bloom.

• Reduce the light during the cycling phase. The light should be reduced (6 hours per day, lower intensity) during the cycling phase, and the light can be increased to the normal level once the cycle is complete and the plants or the corals are growing.

• Be patient. The new tank syndrome is a normal phase, and the patience is the right intervention. The tank that is rushed is the tank that is over-managed, and the over-management can produce more problems than it solves.

Cause 7: The seasonal cause (the pond-specific)

The seventh cause, specific to the outdoor pond. The pond is subject to the seasonal changes in temperature, light, and nutrient availability, and the pond develops the seasonal algae blooms that are characteristic of the spring and the summer. The spring bloom (the "spring diatom bloom") is often the first major pond event of the year, and the summer bloom (the "green water" or "pea soup" bloom) is often the most dramatic.

The signal pattern. The algae bloom develops in the spring (as the temperature and the light increase) or in the summer (as the nutrients and the light peak), and the bloom resolves in the fall (as the temperature drops and the light decreases).

The fix. Manage the seasonal cycle. Specifically:

• The UV sterilizer is the most effective mechanical fix for the pond. The UV sterilizer is sized for the pond's flow rate (the UV sterilizer should treat the entire pond volume once per hour), and the UV sterilizer is on continuously during the warm months.

• The barley straw is a biological control that works slowly. The barley straw (placed in a mesh bag in the pond, with the recommended dosage per gallon) produces hydrogen peroxide as it decomposes, and the hydrogen peroxide inhibits the algae growth. The barley straw takes 2–4 weeks to start working, and the barley straw is most effective as a preventive (added in the early spring) rather than as a treatment for an existing bloom.

• The pond plants (water lilies, water hyacinths, submerged plants) compete with the algae for the nutrients. The pond that has a substantial plant mass (at least 50% of the pond surface covered with floating or emergent plants) is a pond that has less algae.

• Reduce the feeding and the nutrient input during the bloom. The pond that is being overfed or that is receiving runoff from the surrounding landscape is a pond that is providing the algae with excess nutrients.

The four mistakes that turn a manageable bloom into a chronic crisis

Mistake 1: Treating the algae instead of the cause. The most common mistake. The owner treats the algae with an algicide, the algae dies, the algae returns, the owner treats again, the cycle continues. The algicide treats the symptom. The algicide does not treat the cause. The fix is to identify and address the underlying cause, and to use the algicide only as a temporary measure while the underlying cause is being addressed.

Mistake 2: Adding more light to "help the plants." The second mistake. The owner sees the algae, the owner assumes the plants are not getting enough light, the owner increases the light, the algae blooms more. The owner of a high-algae tank should be reducing the light, not increasing it. The fix is to address the underlying cause of the algae (usually the excess nutrients) and to allow the plants to adapt to the existing light level.

Mistake 3: Using algicides in the reef tank. The third mistake. The algicides that are safe for freshwater fish are often toxic to the saltwater invertebrates (the corals, the shrimp, the snails) and to the beneficial bacteria. The algicides in the reef tank can produce a tank crash. The fix is to use the natural methods (the macroalgae in the refugium, the water changes, the reduced feeding) and to avoid the algicides.

Mistake 4: Not addressing the underlying cause after the bloom resolves. The fourth mistake. The bloom resolves (either naturally or with the algicide), the owner assumes the problem is gone, the owner does not address the underlying cause, the bloom returns in 4–8 weeks. The fix is to address the underlying cause before the bloom returns, and to use the bloom as the motivation to make the long-term change.

The bottom line

The algae bloom is a symptom, not a problem. The problem is the underlying condition (excess light, excess nutrients, excess waste, inadequate circulation, inadequate plant competition, new tank syndrome, or seasonal factors) that is allowing the algae to bloom. The fix is to identify the underlying condition, to address the underlying condition, and to use the algicides and the UV sterilizers as temporary measures while the underlying condition is being addressed.

The owner who addresses the underlying cause is the owner who has a tank or a pond that is algae-free in the long term. The owner who treats the symptom is the owner who has a tank or a pond that is in a chronic cycle of bloom and treatment. The difference is the diagnosis. The diagnosis is the framework above. The framework, applied with attention, is the right prevention.

The algae bloom is a normal part of the aquatic ecosystem, and the small amount of algae is a sign of a healthy system. The algae becomes a problem when the bloom exceeds the normal range, and the bloom is a signal that something is out of balance. The fix is the balance restoration. The balance restoration is the framework above. The framework, applied with consistency, is the right prevention.

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About the author

Kenji Tanaka is a pond and aquarium keeper with 20+ years of experience ranging from indoor reef tanks to multi-thousand-gallon koi ponds, and including a wide range of freshwater setups. He has worked with both hobbyists and small commercial aquaculture operations on algae control, water chemistry, and long-term system design, with a particular focus on the practical realities of maintaining a healthy aquatic ecosystem. He writes regularly on the day-to-day realities of keeping the water clear and the system stable.