Live Food for Marine Fish Fry: From Rotifers to Copepods

Raising marine fish fry is one of the trickier challenges in the aquaculture hobby, and almost every failure traces back to food. The fry have small mouths, fast metabolisms, and instinctive rejection of anything that doesn't move correctly.

This guide walks through the standard live food pipeline used by hobbyist and commercial breeders alike, from hatch through settling.

The pipeline overview

Most marine fry follow a similar feeding progression based on mouth size:

• Hatch to ~day 5. Tiny mouths. Need rotifers (50 to 200 microns).
• Day 3 to ~day 14. Mouth size growing. Rotifers plus copepod nauplii.
• Day 7 to ~day 21. Larger prey accepted. Enriched live brine shrimp nauplii (Artemia).
• Day 14 onward. Frozen and dry foods become viable as fry settle and grow.

The exact timing varies by species. Clownfish move through the pipeline faster than dottybacks. Mandarins and anthias take longer.

Rotifers: the foundation

Rotifers (Brachionus plicatilis and B. rotundiformis) are the workhorse of marine fry rearing. They're small enough for almost any first-feeding larva, easy to culture, and dense cultures can support hundreds of fry per litre.

Key points:

• Cultures must be fed live phytoplankton continuously to maintain rotifer nutritional quality.
• The greenwater technique (slightly green water in the larval tank) helps fry hunt rotifers and keeps the rotifers nutritionally strong.
• Rotifers must be enriched with phyto for at least 6 to 12 hours before being fed to larvae.
• Density in the larval tank should be 5 to 10 rotifers per millilitre. Too few and the fry can't find them. Too many and water quality crashes.

Copepod nauplii: the upgrade

Copepod nauplii (newly hatched copepods) are nutritionally superior to rotifers. Higher in DHA, EPA, and ARA. The downside: harder to produce in the volumes needed for large larval batches.

The most useful species for fry rearing:

• Apocyclops panamensis nauplii. Small enough for most first-feeding fry, high in fatty acids.
• Parvocalanus crassirostris nauplii. The smallest commonly cultured species, ideal for difficult fry like dottybacks and damsels.
• Acartia nauplii. Slightly larger, used for fry that have outgrown the smallest sizes.

Many commercial hatcheries now use copepod nauplii from day one for species that won't take rotifers, particularly mandarins, anthias, and several wrasses.

Live brine shrimp nauplii (Artemia)

Newly hatched brine shrimp (Artemia nauplii) are the workhorse for fry once they outgrow rotifers. They're easy to hatch, cheap, and provide good size variety.

Critical: brine shrimp nauplii must be enriched before feeding. Plain Artemia are nutritionally hollow and will starve fry over weeks even if the fry eat constantly.

Standard enrichment options:

• Live phytoplankton for 6 to 12 hours before feeding
• Commercial enrichment products (Selco, ReefBoost, etc.) for 12 to 24 hours
• Combination enrichment using both

The transition to dry food

Most fry transition to commercial dry food between day 14 and day 28. The trick is overlapping live food with dry food during the transition, gradually reducing live food density while introducing crushed pellets.

Signs the transition is working:

• Fry visibly snap at falling pellets
• Bellies stay rounded between feedings
• Growth rate continues during the transition

If growth stalls, back off and add more live food until the fry are bigger.

Common mistakes

• Feeding under-enriched live food. Plain rotifers, plain brine shrimp, even plain pods are nutritionally inadequate for fast-growing fry.
• Letting prey density drop too low. Fry stop hunting if they have to work too hard to find food. Maintain density.
• Greenwater that's too dark. Phyto in the larval tank should be lightly tinted, not opaque. Heavy phyto reduces visibility for hunting.
• Skipping the copepod stage. Some species (anthias, mandarins, dottybacks) won't reliably take rotifers. Plan for copepods from day one.

The bottom line

The rotifer to copepod to brine shrimp pipeline is the standard for a reason. Build cultures of all three before you have fry, not after. Larval rearing happens fast and the system has to be ready when the eggs hatch.

If you're new to breeding, start with clownfish. They're the most forgiving species in the hobby and a great way to learn the food chain before tackling harder larvae.