Choosing Microalgae for Live Feed: Size and Ease of Culture

6 min read · Updated 15 October 2026

The best microalga for live feed is the one your animals can eat, that you can keep alive, and that you can produce reliably. For most hobby and small-scale freshwater use that points to Chlorella vulgaris: small round cells, fast growth and a wide tolerance for conditions. Spirulina is better where you want an easy-to-harvest, protein-rich feed for larger animals. Marine hatcheries use a different set of species again, such as Nannochloropsis, Isochrysis and Tetraselmis.

The four questions that decide it

1. Can the animal eat it? Cell size and shape have to fit the feeding animal. Filter feeders such as rotifers and Daphnia ingest particles of roughly 2 to 20 micrometres.
2. Can the animal digest it? Some algae have walls that pass through undigested. Soft-walled algae give more nutrition per cell.
3. Does it match your water? Freshwater animals need a freshwater alga. Marine species need a marine strain, or at least tolerance of salt.
4. Can you keep it going? A species that crashes every few weeks costs more than it gives.

Comparison of common choices

Ranges vary by strain and source, so treat these as starting points. The marine species are listed because hatcheries often use them, but they need seawater-based media and are outside what most hobbyists keep.

Chlorella: the all-round choice for fresh water

Chlorella is small enough for rotifers, Daphnia and many fry, divides quickly (doubling in roughly 8 to 24 hours under good conditions), and tolerates a range of temperatures and light levels. Its main disadvantage is the tough cell wall, which makes it less useful as a direct food for animals that lack strong digestion. See algae cell walls for the details, and feeding rotifers with Chlorella for how it is used.

Spirulina: the practical filament

Spirulina has soft walls and a high protein content, so it digests easily. Because its filaments are large, it is a poor match for tiny filter feeders but a good one for adult brine shrimp, larger Daphnia, snails and shrimp. It also grows in alkaline water that few contaminants tolerate, making it forgiving. The trade-off is warmth: it wants 30 to 35 °C. Spirulina culture basics covers what it needs.

What to match to what

• Rotifers and very small filter feeders: Chlorella.
• Daphnia: Chlorella, or a mix with a little Spirulina.
• Fry in green water: Chlorella for the tank water; see green water for fry.
• Larger invertebrates and snails: Spirulina.

A worked example: choosing for three different projects

Project 1: a breeding tank for small tetras

The goal is to feed rotifers or infusoria for first-feeding fry, and to tint the water for the first two weeks. The fry have mouths of about 150 micrometres. Chlorella is the clear choice: small, freshwater, tolerant of room temperature and it feeds rotifers directly. You need a jar of green water at around 20 to 30 million cells per millilitre as a feed stock.

Project 2: a shrimp tank with a few fan shrimp

The animals filter suspended particles of a few micrometres to a few tens of micrometres. A thin Chlorella suspension suits them. Spirulina paste also works as a settled food for the grazers and snails. Use both in small amounts, as in feeding filter feeders in a home tank.

Project 3: a school science display

Here the goal is visibility and ease. Chlorella grows fast and shows a visible change within days, so students see results. Spirulina is more interesting under the microscope because of its spirals. A pair of small jars, one of each, gives a good comparison, as in Spirulina vs Chlorella.

Other traits worth weighing

• Doubling time. Faster species recover from harvest sooner. Chlorella can double in 8 to 24 hours; Spirulina in one to three days.
• Light need. Both prefer moderate to high light, but Spirulina bleaches in very strong sun if the culture is thin.
• Tolerance of mistakes. Chlorella forgives a missed day better than most; Spirulina forgives contamination better because of its alkaline medium.
• Harvest effort. Spirulina is strained in a minute; Chlorella needs a settling step, covered in how to harvest algae.
• Storage. Both can be held as a small, dim, cool reserve for weeks.

Beyond the two: a note on marine species

For marine rotifers and larvae, hatcheries use Nannochloropsis, Isochrysis and Tetraselmis. They are grown in seawater-based media, sometimes at 20 °C, and often need lower light and tighter hygiene. If you keep a reef or marine breeding tank, buy those species from a supplier that sells them live, and treat them as a separate project. The principles of size, digestibility and ease still apply.

Mixing species for feeding

Mixed diets usually give better results than single species. A simple approach is to keep two jars of different algae and mix them at feeding time, perhaps 70% Chlorella and 30% Spirulina for freshwater filter feeders. Observe whether the animals eat both and adjust. Log the mix and the response, as in the culture log guide.

Common questions

Can I mix Chlorella and Spirulina in one jar?

They want different pH and temperature, so one will outcompete the other. Grow them separately and mix at feeding time.

Which is easier for a beginner?

Chlorella tolerates a wider range of room conditions. Spirulina is easier to harvest and to keep clean, but needs warmth.

Can I use pond water as a starter?

You can, but you cannot control what is in it, and predators and competitors come with it. A defined starter culture is more reliable.

How do I know if an animal is eating the algae?

Watch the water clear over hours, and look at the animals under a magnifier: a green gut is a sign of feeding.

Is a bigger alga more nutritious?

Not by size. Nutrition depends on composition and digestibility, not on cell size.

Keep reading

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Green Water in Larval Rearing: What It Is and Why It's Used

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