How to culture Nannochloropsis oculata at home
Grow your own live phytoplankton from a single starter culture. This guide covers everything from first setup and lighting through to harvesting, storage, and keeping your culture running indefinitely.
What you need
Phytoplankton culture is different from copepod or rotifer culture in one important way: light is the primary driver of growth, not food. Nannochloropsis oculata is a photosynthetic organism. It uses light energy to convert nutrients and carbon dioxide into new cells. Your job is to provide the right light, the right nutrients, and clean conditions. The biology does the rest.
- Culture container (4L clear container included in kit)
- Air pump with airline tubing
- Airline suction cup (included in kit)
- Daylight-spectrum light source (LED strip, fluorescent tube, or desk lamp)
- Timer for lighting control (16 hours on, 8 hours off)
- Reefphyto Guillard's f/2 nutrient (included in kit)
- 20ml dosing syringe (included in kit)
- Clean synthetic seawater at 1.025 SG
Unlike copepod and rotifer cultures, phytoplankton absolutely requires a dedicated light source. A cool white or daylight-spectrum LED strip or fluorescent tube positioned 10 to 15cm from the container, running 16 hours on and 8 hours off on a timer, is the simplest and most effective setup. Avoid warm-white bulbs and keep the culture out of direct sunlight, which causes temperature spikes and promotes unwanted algae growth.
Setting up your culture
The initial setup takes about 15 minutes. Cleanliness is critical with phytoplankton because you are creating an environment where a single species of microalgae should dominate. Any contamination introduced during setup can compete with or overwhelm your Nannochloropsis before it establishes.
Sterilise the culture container before first use. A dilute bleach solution works well. Rinse thoroughly with clean fresh water until no bleach residue remains, then rinse once more with your prepared saltwater. This is the one culture where sterilisation genuinely matters at setup.
Fill the vessel
Add 3.5 litres of clean synthetic seawater at 1.025 SG to the sterilised container. Use reverse osmosis water mixed with quality reef salt. Tap water introduces contaminants that can inhibit growth.
Add f/2 nutrient
Using the dosing syringe, add 6ml of Reefphyto Guillard's f/2 nutrient to the saltwater. This provides the nitrogen, phosphorus, trace elements, and vitamins that Nannochloropsis needs to grow.
Add the starter culture
Pour the entire 250ml Nannochloropsis oculata starter culture into the vessel. The water will turn a light green immediately. This is your starting point.
Set up aeration
Connect the airline to the air pump and secure it to the container wall with the suction cup. Submerge the open end near the bottom. Set to gentle, steady bubbling. Aeration keeps cells suspended and supplies CO2 for photosynthesis.
Position the light
Place your daylight-spectrum light source 10 to 15cm from the container. Set a timer for 16 hours on, 8 hours off. Consistent photoperiod is more important than light intensity.
Cover loosely
Place a loose cover over the top to prevent dust and contamination while allowing gas exchange. Do not seal airtight. The culture needs the CO2 supplied by aeration and ambient air.
The initial 6ml of f/2 provides everything the culture needs until the first harvest. Adding more nutrient during this phase can cause a culture crash. Wait until you harvest and dilute before adding the next dose.
Growth stages and colour
With phytoplankton, colour is everything. The colour of your culture tells you how dense it is, whether it is growing well, and when it is ready to harvest. Learning to read colour is the single most useful skill in phytoplankton culture.
Days 1 to 3
The culture will be a light, thin green. It may look like nothing is happening. This is normal. The cells are adapting to the new environment and beginning to divide. Do not adjust anything during this period.
Days 3 to 7
The green deepens noticeably. The water becomes more opaque as cell density increases. You should see the colour change day by day. This is the exponential growth phase where the population is doubling rapidly.
Days 7 to 10
The culture reaches a dense, dark green. It should be thick enough that you cannot see clearly through the container. This is harvest density. The culture is ready.
Growth speed depends on light intensity, temperature, and nutrient availability. Some cultures reach harvest density in seven days, others take twelve. Do not harvest based on a fixed timeline. Harvest when the culture is a consistent, dense, dark green throughout the vessel.
Target parameters
Nannochloropsis oculata is tolerant of a range of conditions, but hitting these targets gives you the fastest growth and densest cultures.
Salinity and water quality
Unlike rotifers, which culture best at lower salinity, Nannochloropsis oculata grows well at standard reef salinity of 1.025 SG. Use reverse osmosis water mixed with synthetic marine salt. Tap water introduces chlorine, chloramine, and dissolved solids that inhibit growth or promote contamination.
The f/2 nutrient included in your kit is specifically formulated for marine microalgae and provides the correct ratios of nitrogen, phosphorus, and trace elements. Do not substitute with general-purpose plant fertilisers - the formulation matters.
Harvesting
The harvesting method for phytoplankton is fundamentally different from copepods or rotifers. Instead of straining organisms from the water, you are dividing the culture itself. The technique is called semi-continuous harvesting and it is what allows a single culture to run indefinitely from one starting point.
How it works
When the culture reaches harvest density (dense, dark green), remove approximately 70 percent of the volume. This is your harvested phytoplankton, ready to use. The remaining 30 percent stays in the container as the seed for the next cycle. Top up the container with fresh saltwater, add f/2 nutrient at the recommended rate, and the culture begins growing again immediately. Within 7 to 10 days it is back to harvest density and ready for the next harvest.
What to do with the harvest
The harvested 70 percent can be used immediately or stored. Feed it directly to your reef tank for coral and filter-feeder nutrition. Use it to feed your copepod or rotifer cultures. Add it to a brine shrimp enrichment vessel. Or refrigerate it for later use.
Storing harvested phytoplankton
Place the harvested phytoplankton in a clean, sealed container in the refrigerator at 4 to 8 degrees Celsius. Shake gently every one to two days to prevent cells settling and dying at the bottom. Refrigerated phytoplankton remains viable for two to three weeks. Use within this window for best results.
Always leave at least 30 percent of the culture in the vessel as your seed stock. Harvesting more than 70 percent extends recovery time and increases the risk of contamination during the regrowth phase when cell density is low. Stick to 30/70 and the culture stays healthy and productive indefinitely.
Maintenance
Phytoplankton cultures are the lowest-maintenance of the three culture types. There is no daily feeding to manage, no fortnightly deep clean, and no waste buildup to worry about. The culture looks after itself between harvests. Your main responsibilities are light consistency and clean technique at harvest time.
Daily (30 seconds)
Glance at the culture. Is the light on? Is the air pump running? Is the colour getting darker? That is all you need to check between harvests. There is nothing to feed, nothing to test, nothing to adjust.
At each harvest (every 7 to 14 days)
Remove 70 percent. Add fresh saltwater. Add f/2 nutrient. Resume the light cycle. This is the only active maintenance step, and it takes under five minutes.
Periodically
Every three to four harvest cycles, consider starting a completely fresh culture from your existing stock rather than continuing to top up the same vessel. Pour 30 percent of the culture into a freshly sterilised container with new saltwater and f/2, and discard the old vessel contents. This prevents gradual contamination buildup that can reduce growth rates over many cycles.
The simplest insurance against losing your culture is to maintain a second small vessel (even a clean glass bottle on a windowsill with some f/2 and a splash of culture) as a backup. If your main culture ever crashes or becomes contaminated, the backup restarts you immediately without needing to order a new starter.
What your Nannochloropsis feeds
Nannochloropsis oculata sits at the very base of the marine food chain. At 1 to 5 microns, it is small enough to be consumed by the tiniest filter feeders and zooplankton, and its high EPA (eicosapentaenoic acid) content makes it one of the most nutritionally valuable microalgae species available to hobbyists.
Copepod and rotifer cultures
Live Nannochloropsis is the preferred feed for both copepod and rotifer cultures. It keeps zooplankton populations producing consistently and improves the nutritional quality of the copepods and rotifers you harvest for your fish and corals.
Corals and filter feeders
SPS and LPS corals, soft corals, fan corals, tube worms, feather dusters, and bivalves all benefit from regular phytoplankton dosing. Nannochloropsis provides the fatty acids, chlorophyll, and micronutrients that support calcification, polyp extension, and overall health.
Brine shrimp enrichment
Newly hatched brine shrimp can be gut-loaded with Nannochloropsis before feeding to fish or larvae, dramatically improving their nutritional value.
Troubleshooting
If your culture is not greening up, has changed to an unusual colour, has developed a foul smell, or has gone cloudy rather than green, head to our dedicated troubleshooting centre. It covers every common phytoplankton culture problem with step-by-step guidance.
Culture Troubleshooting Centre
Diagnose and fix phytoplankton, copepod, and rotifer culture problems with Darren's step-by-step guidance.
If you cannot find the answer there, contact me directly. I am happy to troubleshoot culture issues personally.
Download this guide
Want a printed copy to keep next to your culture vessel? Download the complete guide as a PDF.
Download PDF GuideShop Phytoplankton Culture Kit
Phytoplankton is the foundation of everything else in the live food chain. Once you have a Nannochloropsis culture running, your copepods are better fed, your rotifers are more nutritious, your corals get daily live nutrition, and the whole system becomes more self-sustaining. It is the single most impactful culture you can run. If you have any questions, get in touch.
Darren
Founder, Reefphyto Ltd - Wales, UK - Est. 2008