How light-induced algae growth inside RAS tanks is becoming an underestimated operational challenge

Recirculating aquaculture systems (RAS) are engineered to maintain precise biological and chemical conditions, cycling water through filtration, aeration, and treatment stages to sustain healthy fish populations in enclosed environments. The degree of control these systems offer is their defining advantage, but that same closed-loop architecture creates conditions where unintended biological growth can establish itself and compound rapidly. Among the less-discussed operational challenges facing RAS operators today, light-induced algae growth stands out as one that frequently develops below the threshold of routine monitoring until its effects on system performance become difficult to ignore.

This article examines algae growth in RAS tanks from the ground up, starting with what it is, moving through the biological mechanisms that drive it, and arriving at practical approaches to lighting design and management that reduce contamination risk. Each section builds on the last, so readers who are new to this challenge will develop a complete operational picture by the end.

What Light-Induced Algae Growth in RAS Tanks Is

Light-induced algae growth in RAS tanks refers to the colonisation and proliferation of photosynthetic microorganisms, primarily microalgae and cyanobacteria, within the enclosed water volumes, tank walls, pipework, and filtration media of a recirculating aquaculture system, driven by the presence of artificial or natural light. Unlike algae blooms in open-water aquaculture, where dilution and environmental variability limit growth, RAS environments provide algae with a remarkably stable habitat: consistent temperature, abundant dissolved nutrients, and, in many installations, continuous or semi-continuous light exposure.

The term “light-induced” is important here, because it distinguishes this category of algae growth from chemically or thermally driven proliferation. The primary trigger is photosynthesis, the process by which algae convert light energy and dissolved carbon dioxide into biomass. Without sufficient light, most algae species cannot sustain meaningful growth rates regardless of nutrient availability. In RAS tanks, the introduction of light, whether from overhead production lighting, natural daylight entering through transparent panels, or photoperiod lamps used to manage fish behaviour, is the activating variable that transforms a nutrient-rich aquatic environment into a viable algae habitat.

For example, a salmon smolt facility using photoperiod lighting to suppress early maturation may inadvertently illuminate tank walls and water surfaces at intensities sufficient to support algae colonisation, even when the primary lighting intent is biological rather than operational. This is a common entry point for the problem: lighting decisions made for fish management purposes that carry unintended consequences for system cleanliness.

How Light Penetration Triggers Biological Cascades Inside Closed Systems

Understanding how algae growth escalates from initial colonisation to systemic contamination requires a clear picture of the biological cascade that light penetration initiates inside a closed recirculating system.

The Photosynthetic Activation Threshold

Algae do not require high light intensities to begin growing. Many species found in aquaculture environments are adapted to low-light conditions and can initiate photosynthesis at irradiance levels well below those considered meaningful for fish production lighting. Once light penetrates a water column or reaches a surface that retains moisture and dissolved nutrients, tank walls, filter media surfaces, pipework interiors, algae spores already present in the water begin to fix carbon and reproduce.

The initial growth phase is slow and often invisible to the naked eye. A thin biofilm layer forms on illuminated surfaces, typically appearing as a faint green or brownish discolouration. At this stage, the biomass is small and the system impact is negligible, but the colony is establishing the structural foundation for accelerated growth.

Nutrient Amplification in a Closed Loop

What distinguishes RAS environments from open systems is the concentration of nutrients within a finite, recirculating water volume. Fish metabolic waste, primarily ammonia and nitrates, is continuously introduced into the water column. Biological filtration converts ammonia to nitrite and then to nitrate, but nitrate accumulates unless actively removed through water exchange or denitrification. This nitrate-rich environment is precisely the nutrient profile that supports vigorous algae growth once light is present. In effect, the RAS filtration system that protects fish from ammonia toxicity simultaneously creates an ideal fertiliser supply for algae.

As algae biomass increases, the biological cascade accelerates: algae consume dissolved carbon dioxide during daylight periods, raising pH; at night, algae switch to respiration, consuming dissolved oxygen and releasing carbon dioxide, driving pH downward. This diurnal pH swing, which can be pronounced in systems with significant algae biomass, begins to stress fish and destabilise the carefully managed water chemistry that RAS operators depend on.

Why Algae Proliferation Disrupts Core RAS Processes

Building on the nutrient and pH dynamics introduced above, algae proliferation creates disruption across several interdependent RAS processes simultaneously, which is what makes it a compounding rather than isolated operational problem.

The most immediate impact is on dissolved oxygen (DO) management. RAS systems are engineered to maintain DO within a narrow range suited to the target species. Algae respiration during dark periods creates a competing oxygen demand that is not accounted for in the system’s aeration design. In facilities where algae biomass has reached significant levels, overnight DO drops can exceed the tolerance thresholds of sensitive species, leading to sublethal stress or, in severe cases, mortality events.

Biofilter performance is a second area of disruption. Algae colonising the surfaces of biological filter media, moving bed biofilm reactors (MBBRs), trickling filters, or submerged fixed media, compete directly with nitrifying bacteria for surface area and oxygen. Nitrifying bacteria are slow-growing and sensitive to competition; algae, with the energy advantage of photosynthesis, can outcompete them on illuminated media surfaces, reducing the system’s ammonia processing capacity and elevating ammonia concentrations in the fish-holding environment.

Mechanical filtration is also affected. Algae cells and fragments shed from colonised surfaces pass into drum filters and other mechanical separation equipment, increasing the organic load and filter backwash frequency. In systems not designed for this additional solids burden, the result is reduced filtration efficiency and elevated suspended solids in the fish-holding water, a condition that compromises gill function and increases disease susceptibility.

Identifying Early Warning Signs of Algae Colonisation

Early identification of algae colonisation is significantly more effective than remediation after proliferation has taken hold. The challenge is that the earliest indicators are subtle and easy to attribute to other causes without a systematic monitoring approach.

The following signs, taken together, indicate developing algae colonisation before visible biomass accumulation becomes obvious:

• Unexplained diurnal pH fluctuations, rising pH during light periods and falling pH at night, that are not accounted for by fish respiration or CO2 stripping alone
• Gradual increases in dissolved oxygen variability between light and dark periods, particularly overnight DO reductions that were not previously observed
• Visible green, brown, or reddish-brown surface films on tank walls, particularly in areas receiving direct or reflected light
• Elevated turbidity or greenish tint in water samples drawn from illuminated sections of the system
• Increased drum filter backwash frequency without a corresponding increase in feed input or fish biomass
• Declining biofilter nitrification efficiency, indicated by rising ammonia or nitrite levels without an obvious change in stocking density or feeding rate

A useful diagnostic approach is to compare water quality parameters between illuminated and shaded sections of the same system. If pH and DO variability are consistently higher in areas with greater light exposure, algae photosynthesis is the most probable driver. This comparison can be performed with standard water quality monitoring equipment already present in most RAS facilities, without requiring additional analytical infrastructure.

How Lighting Design Choices Determine Algae Risk Levels

With the biological mechanisms and warning signs established, it becomes clear that lighting design is not a peripheral consideration in RAS facility planning, it is a primary variable in algae contamination risk management.

Spectral Composition and Algae Photosynthesis

Algae are most efficient at absorbing light in the red (approximately 660-700 nm) and blue (approximately 430-450 nm) spectral ranges, the same wavelengths that drive chlorophyll-based photosynthesis. Broad-spectrum white light sources, including many LED arrays used in aquaculture production lighting, emit across these ranges. A facility that selects lighting primarily for human visibility or fish behaviour management without considering spectral output may be inadvertently optimising for algae growth as a secondary effect.

Green-shifted or narrow-spectrum light sources that avoid the red and blue absorption peaks of chlorophyll can reduce photosynthetic efficiency for algae without meaningfully impairing the primary lighting function in many production contexts. This is not a universal solution, some fish species require specific spectral environments for welfare or production reasons, but it represents a design consideration that is frequently overlooked at the facility planning stage.

Light Intensity, Duration, and Directionality

Beyond spectrum, three additional design parameters directly influence algae growth risk: intensity, photoperiod duration, and the directionality of light distribution. Higher intensities reaching tank surfaces and water columns above the algae compensation point, the irradiance level at which photosynthesis exceeds respiration, accelerate growth rates proportionally. Longer photoperiods extend the daily window of photosynthetic activity. Light directed at tank walls, pipework, or filter media rather than the water surface or fish-holding zone creates illuminated attachment surfaces for biofilm formation.

For example, overhead lighting positioned to illuminate the full tank cross-section, a common configuration in recirculating systems with circular or raceway tanks, typically creates significant wall illumination as a byproduct. Shielding or directing fixtures to reduce wall irradiance, while maintaining adequate illumination of the water column for fish welfare, can substantially reduce the surface area available for algae colonisation without changing the total light output of the installation.

Controlling Algae Growth Through Integrated Light Management

Effective algae control in RAS environments is not achieved through any single intervention but through an integrated approach that addresses light exposure at the source, manages the biological environment, and incorporates routine monitoring to detect colonisation before it compounds.

The foundational principle is light exclusion where it is not operationally necessary. Any illuminated surface that does not serve a direct production function, fish observation, worker safety, photoperiod management, is a potential algae colonisation site. Opaque covers on pipework, shaded filter media housings, and light-blocking materials on tank wall sections below the waterline are practical structural measures that reduce the total illuminated surface area in a system without affecting production lighting objectives.

Where light exclusion is not feasible, photoperiod management offers a complementary control mechanism. Algae growth is cumulative, reducing daily light exposure even partially slows the rate of biomass accumulation and extends the interval between cleaning cycles. Timed lighting systems that restrict illumination to periods when human access or fish management requires it, rather than running continuously, can meaningfully reduce algae growth rates in facilities where lighting has previously operated on a fixed or unmanaged schedule.

Routine mechanical cleaning of illuminated surfaces remains a necessary component of any algae management programme, but its frequency and intensity are directly proportional to the algae growth rate, which is itself a function of light exposure. Facilities that address lighting design first find that cleaning intervals extend and the labour burden of surface maintenance decreases correspondingly. The practical implication is that investment in lighting design and management at the facility planning stage reduces the ongoing operational cost of algae control throughout the system’s service life.

Finally, water exchange rates and nitrate management influence the nutrient availability that sustains algae growth. While light is the activating variable, nutrient concentration determines growth rate once algae are established. Systems that maintain lower nitrate concentrations through higher water exchange or active denitrification create a less hospitable environment for algae even where light exposure cannot be fully controlled.

For aquaculture operators seeking guidance on purpose-built lighting solutions designed to minimise unintended biological growth while meeting the visibility and safety requirements of offshore and onshore installations, contact Sabik’s technical team to discuss your facility’s specific requirements.