Spirulina growing: complete beginners guide.

What you need to start

• Growing vessel: Start with a 10–20 L food-grade HDPE container, a glass aquarium, or a clear polypropylene storage box. Transparent or translucent is better than opaque for light penetration. Minimum surface area: 30×20 cm. Avoid galvanised metal (zinc toxicity) or coloured containers that block light.
• Aeration: A standard aquarium air pump (2–5 W) with silicone tubing and an air diffuser stone. Aeration circulates the culture, prevents settling, and provides CO₂from ambient air. This is the minimum viable mixing system for 10–20 L.
• Light: A south-facing windowsill (4+ hours direct sun/day in summer) or a basic LED grow light (5,000–6,500 K, 2,000–5,000 lux at culture surface). 18 hours light / 6 hours dark cycle for artificial light. Do not use UVC sterilisation lamps — these kill spirulina.
• Temperature: 25–35°C for optimal growth. A fish tank heater (25 W for 20 L) set to 30°C is the simplest solution for indoor growing. Below 20°C: growth slows significantly. Below 15°C: growth stops. Above 38°C: culture begins to die.
• pH meter: Non-negotiable. Spirulina grows at pH 9–10.5. Below 9: risk of green algae contamination. Above 11: alkaline damage to culture. A basic digital pH pen (~£10–£20) is sufficient; calibrate with pH 7 and pH 10 buffer solutions monthly.
• Nutrients (Zarrouk medium): Pre-mixed Zarrouk medium salts are available from spirulina supplier shops online. Alternatively, source individual salts (sodium bicarbonate, sodium nitrate, potassium phosphate, magnesium sulphate, iron sulphate with EDTA). The full Zarrouk recipe is in the nutrients guide.
• Starter culture: 1 L of live spirulina culture (dense green, healthy) from an online supplier or an existing grower. Dry spirulina powder will not restart as a live culture — you need live cyanobacterial cells.

Setting up your first culture

• Step 1 — Prepare water: Fill your vessel with 10 L of water. If using tap water: first test for chloramine (ask your water supplier or use a test kit). Chloramine does NOT dissipate on standing — unlike chlorine. If chloramine is present, use activated carbon filtration (pour through a Brita or similar) or add 0.1 g/L ascorbic acid to neutralise it. Plain leaving-to-stand overnight is only sufficient for chlorine, not chloramine.
• Step 2 — Dissolve nutrients: Add Zarrouk medium salts according to the instructions for your specific product. Stir until fully dissolved. Check pH: it should read 9–9.5 naturally with bicarbonate-based medium. If below 9, add a small amount of sodium bicarbonate and stir.
• Step 3 — Add starter culture: Pour 1 L of live culture into 10 L medium. This gives approximately 10% inoculation density. A healthy starter culture will be dark green and odourless (or faintly earthy). If the starter smells sulphurous or has brown patches, do not use it.
• Step 4 — Start aeration and heat: Begin aeration immediately. Set heater to 28–30°C. Place under light or at your window.

Week-by-week: what to expect

• Days 1–5 (lag phase): The culture adapts to its new environment. Growth appears slow; the culture may look slightly paler than the starter. This is normal. Do not add extra nutrients or change conditions — stress during lag phase delays establishment.
• Days 5–14 (exponential growth): Culture density increases visibly. The medium darkens from pale green to a richer, deeper green. Culture volume doubles approximately every 3–5 days under good conditions. Check pH daily — photosynthesis raises pH. If pH reaches 10.5: add CO₂ by bubbling, or gently buffer with a few drops of weak acid (or add more CO₂-generating nutrient concentrate).
• Days 14–21 (density build): Culture becomes distinctly dark green and increasingly opaque. When a white strip of paper held 1 cm behind the vessel is no longer clearly readable through the culture, you are approaching harvest density.
• Day 21–28 (first harvest): Culture is ready to harvest when it is dark green and the OD₆₅₀ exceeds 1.0 (if you have a spectrophotometer or a basic OD meter) or when the simple visual test shows opacity at 10 cm depth. See the harvest section below.

Your first harvest

• How much to harvest: Remove 20–30% of the culture volume — never more than 30% at once. For a 10 L culture: harvest 2–3 L, then immediately replace with 2–3 L of fresh Zarrouk medium at the same temperature. This semi-continuous harvest keeps the culture at productive density without crashing it.
• Basic harvest method: Pour the harvested volume through a fine mesh bag (25–50 µm nylon filter bag, available from home-brewing or aquatic suppliers). The spirulina filaments are retained on the filter; the green liquid passes through and can be returned to the culture vessel (it contains nutrients). Rinse the filtered spirulina once with clean water to reduce salt content.
• Fresh consumption: Fresh wet spirulina paste can be added directly to smoothies or food. Refrigerate and consume within 24 hours. Fresh spirulina paste is 80% water; a tablespoon of paste is approximately 2–3 g dry weight equivalent.

The three most common beginner mistakes

• Chloramine in tap water: The single most common cause of culture failure in the UK and many urban areas. Chloramine kills cyanobacteria at trace concentrations. Never assume tap water is safe after standing — test for chloramine first.
• Temperature below 20°C: A windowsill in spring or autumn in the UK can drop to 12–15°C at night. Without a heater, the culture stalls and may not recover when warmed. Use a heater from day one.
• Harvesting too early: Harvesting a thin, pale culture before it reaches density removes too much biomass and crashes growth. Wait for visible dark green opacity before the first harvest — typically 3–4 weeks from inoculation.

Maintaining your culture long-term

• After the first harvest, harvest 20–30% every 2–3 days and replace with fresh medium. This semi-continuous method is the most productive approach for home growers.
• Keep a 1 L sealed stock culture in a separate container at a slightly lower density — this is your backup if the main culture crashes due to contamination or temperature incident.
• Check pH daily, conductivity weekly, and examine a sample under a basic microscope (or a magnifying glass) monthly for signs of rotifer predators or green algae contamination.