How to Culture Phytoplankton at Home for Your Aquarium

Culturing phytoplankton at home is straightforward once you have the right container, a light source, clean saltwater, nutrients, and a starter culture. The entire setup can run on a windowsill or a closet shelf, and a single batch typically reaches harvestable density within one to two weeks. The process is essentially the same one that commercial hatcheries use, just scaled down to bottles or small carboys, and the reward is a steady supply of fresh, living food for corals, clams, copepods, and filter-feeding invertebrates in your reef tank.

Why Bother Growing It Yourself

Bottled phytoplankton from your local fish store works in a pinch, but those products are almost always dead cells preserved in a refrigerated suspension. Live phytoplankton is nutritionally superior because the cells are still metabolically active, producing the fatty acids that make them valuable as feed in the first place. A large-scale analysis of fatty acid profiles across seven major phytoplankton groups found that the production of key omega-3 fatty acids like EPA and DHA varies dramatically by species, and that some of the most commonly cultured groups for aquaculture, such as haptophytes and diatoms, are among the richest producers.¹ When cells die and sit on a shelf, those lipids degrade. Growing your own means you harvest at peak nutritional quality.

Cost is the other motivator. Commercial microalgae production in professional systems runs hundreds of dollars per kilogram of dry biomass, even in optimized setups. A home hobbyist spending a few dollars on nutrient concentrate and using recycled bottles can produce far more phytoplankton than a typical reef tank needs, at a fraction of the cost of buying refrigerated bottles every week.

Picking a Species

Not all phytoplankton are equally easy to grow or equally useful. For a home aquarist, three genera cover most needs:

Nannochloropsis: A tiny green cell packed with EPA, the omega-3 fatty acid that corals and copepods thrive on. It tolerates a wide range of conditions, grows fast, and is arguably the most forgiving species for beginners. Nannochloropsis reached the highest biomass among three tested microalgae at 25 °C, with cell counts roughly three times what it achieved at lower temperatures.²
Isochrysis: A golden-brown flagellate rich in DHA, which is especially important for larval fish and bivalves. It prefers slightly warmer water and can be a bit touchier about contamination, but it grows well in standard culture media.
Tetraselmis: A larger green cell often used to feed rotifers and brine shrimp. It is hardy and fast-growing, and research on Tetraselmis chui showed that both light intensity and day length significantly affect its final cell density, giving you useful levers to control growth speed.³

If your main goal is feeding corals and filter feeders directly, Nannochloropsis is the go-to starter. If you are raising copepods or rotifers as an intermediate food source for fish, you might want Isochrysis or Tetraselmis alongside it. Starter cultures are widely available from aquaculture supply companies and some reef-keeping retailers, usually shipped as a small bottle of live green or golden water.

Equipment and Setup

You do not need a laboratory. The essentials are surprisingly simple:

Containers: Clean, clear bottles work well. Two-liter soda bottles are popular because they are free, transparent, and the right size for a single batch. Glass Erlenmeyer flasks or narrow-mouth jars are a step up. Avoid anything that previously held soap or chemicals.
Light source: A simple daylight-spectrum LED strip or a compact fluorescent bulb. You want something in the 5000–6500 K color temperature range, positioned a few inches from the bottle.
Air pump and tubing: A small aquarium air pump with rigid airline tubing pushed down into the bottle keeps the culture aerated and in suspension. Phytoplankton that settles to the bottom gets starved of light and dies. The bubbling also supplies dissolved CO₂, which the cells need for photosynthesis.
Nutrient solution: A commercial phytoplankton fertilizer (often sold as “f/2 medium” or a proprietary blend based on it) provides the nitrogen, phosphorus, and trace metals the cells require.
Saltwater: Either natural seawater filtered through a fine mesh or synthetic saltwater mixed to the right salinity. More on this below.

Sterilization matters more than most beginners expect. Bacteria, ciliates, and wild algae are everywhere, and a contaminated culture will crash within days. Before starting, rinse your bottles and tubing with a dilute bleach solution, then rinse thoroughly with freshwater, and ideally follow with a rinse of dechlorinated water. Some growers microwave their saltwater for a minute or two to knock back bacteria, though this is impractical at larger volumes.

Preparing Culture Medium

The nutrient recipe your phytoplankton lives on is called the culture medium. The most widely used formulation in both research and hobby circles is Guillard’s f/2 medium, a mix of sodium nitrate, sodium phosphate, trace metals, and vitamins dissolved in seawater. You can buy it premixed from aquaculture suppliers as a liquid concentrate, which is by far the easiest route for a home grower. A few drops per liter of saltwater is typically all it takes.

Conway medium is another common recipe, and research comparing the two across six genera of marine microalgae found that different species actually prefer different media. Tetraselmis, Dunaliella, and Pavlova sustained growth longer in f/2, while Dunaliella, Chlorella, and Isochrysis reached higher peak cell densities in Conway medium.⁴ For a beginner growing Nannochloropsis or Tetraselmis, f/2 is the standard recommendation and works reliably. If you ever branch into Isochrysis and find your cultures plateauing early, experimenting with Conway medium is worth trying.

Getting the Light Right

Phytoplankton are photosynthetic, so light is the engine that drives your culture. But more is not always better, and getting the balance right makes a noticeable difference in how quickly your bottles turn dense green.

Research on Tetraselmis chui showed that higher light intensity produced significantly greater final cell numbers, with cultures grown under the strongest tested light reaching roughly double the density of those grown under the weakest.³ Photoperiod also mattered independently. In practical terms, most home growers run their lights on a timer set to 16–18 hours on and 6–8 hours off, which mimics a long summer day and keeps photosynthesis running without stressing the cells with constant illumination.

A single LED shop light or a strip of cool-white LEDs mounted a few inches from the bottle is plenty. You do not need expensive grow lights or PAR meters. If the culture is turning green within the first few days, your light is adequate. If it stays pale after a week, move the light closer or add a second source.

Temperature and Salinity

Most commonly cultured marine phytoplankton grow well at room temperature, roughly 20–25 °C (68–77 °F). Species differ in their optimum: Nannochloropsis produced its highest biomass at 25 °C, while Chlorella vulgaris peaked at 20 °C in the same study.² The sweet spot for a mixed hobby setup is around 22–24 °C. Avoid placing your bottles near a window that gets direct afternoon sun, which can overheat the culture and kill it in hours.

An interesting wrinkle is that lower temperatures shift the fatty acid composition of the cells. Cooler conditions tend to promote more polyunsaturated fatty acids (the “good” omega-3s), while warmer conditions push cells toward producing more saturated fats.² If you are growing phytoplankton specifically for its nutritional value, keeping the culture on the cooler side of comfortable (around 20 °C) may give you a better fatty acid profile, though growth will be slower.

For salinity, mix your saltwater to a specific gravity of about 1.020–1.025, which is typical seawater range. Marine phytoplankton can tolerate a fairly wide salinity window. A study of four planktonic algae species found they all tolerated short-term exposures to salinities ranging from 5 to 35 parts per thousand, though lower salinities caused more variable stress responses.⁵ The practical takeaway is that your culture is not going to crash from minor salinity fluctuations, but aim for consistency rather than testing the limits.

Understanding the Growth Cycle

Every batch culture follows a predictable arc. When you inoculate a fresh bottle with a starter dose of phytoplankton, the cells go through a brief lag phase while they adjust to the new environment. Then comes exponential growth, where the population doubles at a steady rate and the bottle turns progressively greener. Eventually, the cells start to shade each other and nutrients begin running low, shifting the culture into a phase of declining growth, sometimes called the linear growth phase, where cells are still dividing but more slowly.⁶ Finally, the culture reaches a stationary phase where growth and death roughly balance.

The ideal time to harvest is during late exponential growth or early in the declining phase. The culture is dense enough to be useful but has not yet started degrading. In practice, you learn to read the color: a rich, opaque green (or golden-brown for Isochrysis) that you cannot see through means the culture is dense and ready. Once you harvest about half the volume, top it off with fresh saltwater and nutrients. This is the basic split-and-feed cycle that keeps a continuous supply going.

Many home growers maintain two or three bottles on a staggered schedule. When bottle A is ready to harvest, bottle B is mid-growth, and bottle C was just inoculated. This rotation ensures you always have phytoplankton on hand and reduces the risk of losing everything to a single crash.

Keeping a Backup in the Fridge

One of the simplest insurance policies against culture crashes is to keep a small backup bottle in the refrigerator. Cold temperatures slow cell division almost to a standstill, essentially putting the culture into suspended animation. Research on two species of benthic diatoms found that cultures stored at 4 °C in the dark maintained full cell viability for up to four weeks, with no measurable loss in growth rates when brought back to normal conditions.⁷ The only changes observed were in dry weight and lipid content after three to four weeks, meaning the cells were still alive and ready to grow but had burned through some of their stored energy.

For a home grower, this means you can set aside a small, sealed bottle of dense culture every week or two as a restart kit. If your main bottles crash from contamination or a heat spike, you pull the backup out of the fridge, let it warm to room temperature, add it to fresh medium, and you are back in business within a few days rather than waiting for a new starter culture to arrive in the mail.

Feeding Your Tank

How you dose phytoplankton into your aquarium depends on what you are feeding. For a reef tank with soft corals, gorgonians, and filter-feeding invertebrates like feather dusters or clams, you can pour a small amount of dense culture directly into the tank after turning off your protein skimmer. Start with a modest dose and observe whether the water clears within an hour or two, which indicates the organisms are consuming it. Gradually increase until you find the amount your tank’s inhabitants consume without leaving the water persistently cloudy.

If you are growing phytoplankton to feed an intermediate organism like copepods or rotifers, you pour the culture into the copepod or rotifer container instead. The zooplankton graze on the phytoplankton cells, and you then harvest the enriched zooplankton to feed your fish. This two-step food chain is especially valuable for raising larval fish, which need tiny, nutritious live prey in the first days after hatching. The fatty acid content of the phytoplankton passes up the chain, so starting with a species rich in EPA or DHA, like Nannochloropsis or Isochrysis, means your copepods carry those same beneficial fats to your fish.¹

When Things Go Wrong

Culture crashes are the most common frustration for home growers, and they usually come down to one of three causes: contamination, overheating, or nutrient exhaustion.

Contamination is the big one. Bacteria, protists, and rotifers can sneak in through unsterilized equipment, dirty hands, or even airborne dust settling into an open bottle. The first sign is usually a change in smell, from the mild ocean scent of healthy phytoplankton to something swampy or sulfurous. You might also notice the culture turning a brownish or yellowish color instead of vibrant green, or see the cell density drop rapidly over a day or two. Once a culture is heavily contaminated, it is usually faster to dump it, sterilize the container, and start fresh from your backup rather than trying to save it.

In commercial and research settings, harmful algal blooms pose a serious contamination risk. Wild species like Karenia brevis produce potent toxins, though research has shown that clay flocculation can remove the vast majority of both cells and their toxins from seawater.⁸ For a home grower, the simpler prevention is to never use raw, unfiltered seawater as your culture base. Synthetic saltwater mixed from a commercial reef salt eliminates the risk of introducing wild algae or their toxins. If you do collect natural seawater, filter it through at least a one-micron filter and consider microwaving or pasteurizing it before use.

Temperature spikes are the second most common killer. A bottle sitting near a sunny window on a summer afternoon can easily hit 30 °C or above, which is enough to crash many phytoplankton species. Keep your cultures in a temperature-stable spot, ideally an interior shelf or closet with controlled lighting rather than relying on natural sunlight.

Growing Multiple Species Together

It is tempting to mix two or more phytoplankton species into one bottle for a more nutritionally diverse feed. In practice, mixed cultures rarely stay balanced for long. Competition for nutrients tends to favor one species, which gradually crowds out the others. More intriguing, some phytoplankton species engage in chemical warfare. Research has demonstrated that when certain green algae are grown together, one species can release chemical compounds that inhibit the other’s growth, a phenomenon called allelopathy.⁹

What makes this trickier is that allelopathic interactions are not always present from the start. In one set of experiments, an isolate of Coelastrella did not produce growth-inhibiting compounds when grown alone, but began producing them specifically in response to the presence of Chlorella sorokiniana in co-culture.¹⁰ The chemicals were triggered by competition, not produced constitutively. This means two species might seem to coexist fine for a few days before one starts suppressing the other, leaving you with a monoculture anyway.

The practical advice is to grow each species in its own bottle and mix them at feeding time. You get the nutritional diversity you want without the unpredictable dynamics of interspecies competition. A rotation of Nannochloropsis for EPA and Isochrysis for DHA, grown in separate containers and dosed together into the tank, gives your corals and filter feeders a balanced fatty acid profile without the headache of trying to keep both species happy in one vessel.

Scaling Up Without Losing Control

Once you have the basic bottle method working, you might want to increase your output. The easiest scale-up is simply more bottles, but some growers move to five-gallon carboys, food-grade buckets fitted with clear lids, or even purpose-built photobioreactors made from acrylic tubing. The principles stay the same at any scale: keep the cells in suspension with aeration, provide enough light to penetrate the culture, maintain clean conditions, and refresh nutrients regularly.

The main challenge at larger volumes is light penetration. A two-liter soda bottle is thin enough that light reaches most of the cells. A five-gallon carboy is not. Cells in the center of a large container live in near darkness, which slows overall growth and can lead to die-off in the interior. The research term for this is self-shading, and it is the primary reason cultures transition from exponential growth to a slower linear phase even when nutrients are still available.⁶ To compensate, use containers that are tall and narrow rather than wide, wrap them with LED strips, and increase aeration to keep cells circulating through the illuminated outer layer.

Some hobbyists build continuous-culture systems where fresh medium drips in at one end and dense culture overflows at the other, maintaining the culture perpetually in its fastest growth phase. These work well but require more plumbing and monitoring. For most reef keepers, a staggered rotation of three to six bottles provides more phytoplankton than they can use, without the complexity of a continuous system.

1
PubMed Central. Fatty Acid Profiles and Production in Marine Phytoplankton
2
PubMed Central. Effect of Temperature on Biological Macromolecules of Three Microalgae and Application of FT-IR for Evaluating Microalgal Lipid Characterization
3
Aquaculture. Photoperiod and light intensity effects on growth and utilization of nutrients by the aquaculture feed microalga, Tetraselmis chui (PLY429)
4
Bioresource Technology. Effect of Conway Medium and f/2 Medium on the growth of six genera of South China Sea marine microalgae
5
Estuarine, Coastal and Shelf Science. Repercussions of salinity changes and osmotic stress in marine phytoplankton species
6
Algal Research. Transitional-state growth kinetics of Thalassiosira pseudonana during self-shading in batch culture
7
Aquacultural Engineering. The effect of cold storage on cell viability and composition of two benthic diatoms
8
Harmful Algae. Removal of harmful algal cells (Karenia brevis) and toxins from seawater culture by clay flocculation
9
Oikos. Allelopathy prevents competitive exclusion and promotes phytoplankton biodiversity
10
Algal Research. Evidence for induced allelopathy in an isolate of Coelastrella following co-culture with Chlorella sorokiniana