Fry killing each other: tank colour and shelter change the outcome
Published by Planète Pets · managing editor: Clément Lacaille · how our guides are written
For many species, the very first fry growing up together aren't threatened by disease or bad water chemistry, but by their own siblings: cannibalism among juveniles of slightly different sizes is a well-documented phenomenon in fish farming, and it responds in part to simple tank setup choices. Our fry guidance is gathered in the fish toys and enrichment section.
Rocks or artificial plants: not the same effect
Sepúlveda-Quiroz, Álvarez-Villagómez, Mendoza-Porras, Peña-Marín, Maytorena-Verdugo, Pérez-Jiménez, Jesús-Contreras, Álvarez-González and Martínez-García published in 2023, in Aquaculture, a study on tropical gar larvae (Atractosteus tropicus), a species known for heavy early-stage cannibalism (see the study). The researchers compared, in pairs and then in groups of ten larvae, the effect of tank wall colour and shelter type (rocks versus artificial vegetation) on the number of attacks observed.
The clear result: having a shelter reduces attacks, both in pairs and in groups, and artificial vegetation outperforms rocks at this. On colour, a white background generated the most attacks in pairs, while yellow and purple sharply cut them; in groups, by contrast, pink, blue and yellow drove the most aggression. There is no single best colour — it depends on how many fish are present together.
What this means for a fry tank
- Add a shelter from the first days, even something simple (Java moss, a dense artificial plant), rather than a bare tank that makes visual detection between individuals easy.
- Favour dense vegetation over rocks or hard décor when the goal is to limit aggressive contact between similarly sized juveniles.
- Avoid very pale or very high-contrast backgrounds for a large batch, and keep in mind that a result found in pairs doesn't automatically transfer to a group setting.
- Sorting by size as soon as the gap becomes visible remains the most effective complementary lever: the largest fry in a brood is almost always the aggressor, and tank setup dampens the phenomenon without eliminating it.
Frequently asked questions
Does this apply to common aquarium fish fry?
Tropical gar is an extreme case, chosen precisely because its cannibalism is severe and easy to quantify. The general principle — shelter and décor complexity reducing aggression among juveniles — shows up in other species reared in dense groups, but the precise thresholds (colours, materials) may not transfer exactly as measured here.
Is a shelter enough if fry are underfed?
No: hunger remains a major driver of cannibalism in fish larvae. Tank setup is a complement to adequate, sufficiently frequent feeding, not a substitute — see our guide on hatching brine shrimp and feeding fry.
Should the shelter stay once fry have grown?
Nothing rules it out: a structure limiting direct visual contact stays useful as long as individuals in the same batch remain unevenly sized. For the rest of the tank setup, head to our fishkeeping hub.
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This guide is part of Planète Pets’s Fishkeeping universe. Our advice is general in nature: for any health concern, your veterinarian remains the only reference.