Spirulina is the easiest microalga to harvest at home. Its filaments are long enough to be caught by a fine cloth or a mesh of about 30 to 50 micrometres, so you can strain a jar into a paste in a minute and feed it fresh to shrimp, snails or Daphnia. The only equipment you need is a clean cloth, a bowl and a spoon.
When to harvest
Harvest when the culture is dense and a deep blue-green, usually after one to two weeks of growth, and before it fades. A simple density check is to dip a small white card or disc into the jar: if you lose sight of it within 2 to 3 centimetres, the culture is dense. If you can still see it deeper, wait. Harvesting a portion, rather than the whole jar, is better for continuous production. See the growth phases for why.
What you need
- A fine nylon mesh, a fine cotton cloth or a clean fine-weave cloth (tea towel or sieve cloth).
- A clean bowl or jug to catch the liquid.
- A clean spoon.
- Dechlorinated water for rinsing.
Step by step
- Stir the culture gently so the filaments are evenly spread.
- Pour the amount you want through the cloth, held over the bowl. The medium passes through and the filaments stay on the cloth.
- Return the medium. The liquid in the bowl still has nutrients and bicarbonate. Pour it back into the culture jar, topping up with dechlorinated water and a little fresh nutrient as needed.
- Rinse briefly with dechlorinated water if you plan to feed it to animals that are sensitive to high pH, such as some shrimp and fish fry.
- Gather and squeeze the cloth gently into a paste.
- Feed or store. Use fresh. It keeps in the fridge for a day or two, or can be frozen in small portions.
Feeding the harvest
| Animal | How to give it |
|---|---|
| Freshwater shrimp | A pinch of paste dropped in; remove what is uneaten after a few hours |
| Snails | A small smear on a leaf or rock |
| Daphnia | A few drops of diluted paste into the culture, mixed with Chlorella |
| Brine shrimp | Diluted suspension, small amounts |
Start small. Spirulina is rich, and overdosing rots in the tank. Dosing guidance is in safe ways to add algae to a tank.
Keeping the culture going
After harvest, add fresh medium to replace the volume you took, keep the temperature at 30 to 35 °C, and watch the density return over a few days. See Spirulina culture basics. For a comparison with other harvest methods, see how to harvest algae from a healthy culture.
Making a simple harvest frame
A hand-held cloth works, but a small frame makes harvesting faster and cleaner, and costs almost nothing:
- Take a plastic or stainless steel kitchen sieve about 15 centimetres across.
- Line it with a piece of fine nylon mesh or a clean nylon stocking, cut slightly larger than the sieve.
- Stand the sieve on a bowl that is deep enough that the mesh does not touch the liquid.
- Pour the culture in slowly. The medium collects in the bowl; the filaments remain on the mesh.
- Gather the mesh edges and lift out the paste with a clean spoon.
Wash the mesh in warm water straight away, because dried Spirulina is hard to remove. Rinse it in a dilute bleach solution weekly to avoid carrying contamination between batches.
Saving a fresh harvest
| Method | How | Keeps for |
|---|---|---|
| Fresh in the fridge | Airtight container, a thin layer, cold | 1 to 2 days |
| Freezing in portions | Press into an ice cube tray, freeze, bag the cubes | 2 to 3 months |
| Drying in thin sheets | Spread thinly on baking paper at 40 to 50 °C or in a dehydrator until brittle, then crush | Months, if kept dry and sealed |
Frozen cubes are very convenient: drop one into a tank or into a Daphnia culture. They thaw quickly and disperse easily.
Feeding amounts
As a starting point for a small tank of around 50 litres, a pea-sized piece of fresh paste, or one frozen cube, is a generous portion for a group of shrimp and snails. Remove what is left after four to six hours, and do not feed again until it is gone. For Daphnia cultures, a quarter of a teaspoon of dilute paste per 10 litres is plenty. The suitability of your tank is the best guide: see safe ways to add algae to a tank.
Keeping the parent culture healthy after harvest
- Replace the harvested volume with fresh medium at 30 °C and the right alkalinity.
- Do not harvest more than about half of the culture at once.
- Leave a few days between large harvests so the density can recover.
- Log the date and volume, and compare the recovery time in the following days. The culture log guide is here.
Common questions
Can I eat the harvested Spirulina?
Our cultures are sold for aquarium, aquaculture, hobby and educational use and not as food. Use food-grade, labelled products for eating.
Why does the paste smell strong?
Fresh Spirulina has a distinct earthy-sweet smell. A sour smell means it is spoiling; discard it.
How much can I take at once?
Up to about half of a dense culture is fine, as long as you replace the volume with medium.
Can I harvest Spirulina with a coffee filter?
It works but clogs fast and is slow. A fine nylon mesh is better.
Why does the filtrate look slightly green?
Some short filaments and free pigments pass through. This is normal; return the liquid to the culture.
Ready to grow your own?
Live starter cultures, grown with care and shipped ready to thrive.
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Harvesting Chlorella When Feeding Live Food
Nutrients for Algae When Feeding Live Food
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