The Rule of Six — Where Does It Actually Come From? Fishkeeping Science

The rule of keeping fish in groups of six or more is everywhere in the hobby — but where did it actually come from, and does the science support it? In this video I look at the research behind shoaling and schooling fish, what cortisol measurements tell us about stress and welfare, and what four studies — including a 2025 paper published in PRX Life — actually say about group size and fish wellbeing. The takeaway might surprise you: it's not that the rule is wrong. It's that most people are using it incorrectly. 📖 STUDIES REFERENCED • Sadoul, B. and Geffroy, B. (2019) 'Measuring cortisol, the major stress hormone in fishes', Journal of Fish Biology, 94(4), pp. 541-555. https://onlinelibrary.wiley.com/doi/a... • Schumann, S. et al. (2023) 'Social buffering of oxidative stress and cortisol in an endemic cyprinid fish', Scientific Reports, 13, 20579. https://www.nature.com/articles/s4159... • Bessa, E., Sadoul, B., Mckenzie, D.J. and Geffroy, B. (2021) 'Group size, temperature and body size modulate the effects of social hierarchy on basal cortisol levels in fishes', Hormones and Behavior, 136, 105077. https://www.sciencedirect.com/science... • Lin, G. et al. (2025) 'Experimental Evidence of Stress-Induced Critical State in Schooling Fish', PRX Life, 3(033018). https://doi.org/10.1103/nr7p-m4ff • Saxby, A. et al. (2010) 'The effect of group size on the behaviour and welfare of four fish species commonly kept in home aquaria', Applied Animal Behaviour Science, 125(1-2), pp. 70-76. https://www.sciencedirect.com/science... ----------------------------------------- Visit us in store at Finchley Nurseries, North West London www.pondlifeaquatics.co.uk Instagram: @pondlifeaquatics [email protected] ----------------------------------------- TIMESTAMPS 0:00 The rule of six 1:04 What's the difference between shoaling and schooling? 2:30 How do we measure fish stress? Cortisol explained 3:20 Study 1: Schumann et al. (2023) — Italian riffle dace 4:15 Study 2: Bessa et al. (2021) — 72-study meta-analysis 4:49 Study 3: Lin et al. (2025) — Rummy nose tetra critical state 6:07 Study 4: Saxby et al. (2010) — Neon tetras, tiger barbs and angelfish 7:13 My personal experience with pygmy corydoras 8:29 What this tells us about the rule of six 9:16 What we should do differently 10:42 If your multi-species tank is working... 11:15 Don't take this as permission to overstock 11:34 The takeaway ----------------------------------------- Pond Life Aquatics — We Love It Wet