Problems for children
This page continues
Two halves of the biology have pages of their own.
Nitrification, decomposition and roots
Who actually oxidises the ammonia, why tannins are a brake on decomposition, and why root uptake substitutes for leaf uptake instead of adding to it.
Succession, the egg bank and Lake Sevan
The 1912 order of colonisation, why the animals are already in the mud, algae as the other photosynthesiser, and the lake scale version of all of it.
The animals on top
Shoaling is not schooling, and it is a sensory behavior before it is a social one.
The fish are the last layer of this system and the thinnest. They are also the layer the hobby has the most confident numbers about, and the confidence is not earned.
The definitions are precise and the hobby uses half of each. The distinction biologists use comes from a two page note by Tony Pitcher in 19832, expanded in a 1993 chapter3. A shoal is any group of fish that remain together for social reasons; a school is a shoal that is polarised and coordinated.45 Three things get dropped in transmission, and each of them matters.
They are working tools for describing behavior, not a taxonomy of species. An article that treats "shoaler" and "schooler" as species labels has already left the source behind.
Fish crowded at a feeding spot, or all in the coolest corner, or all behind the same rock, are an aggregation, not a shoal. The cause is external, not social.
All schools are shoals by definition, and there is no clear dividing line between the two6. A healthy group shoals loosely most of the day and snaps into a polarised school for a few seconds when startled. Both are normal.
Access note, stated rather than hidden. Pitcher's 1983 note and the 1993 chapter are both paywalled and were not read here. The definition above is quoted from two independent peer reviewed papers that quote it verbatim45 plus a third that paraphrases the 1983 note6. That is strong, and it is not the same as reading the original.
It runs on two senses, and one of them is fragile
A shoal is held together by vision and by the lateral line, the row of exposed hair cell organs down the flank that reads water movement. The classic experiments separated them. Blinded fish with the lateral line intact still held position in the school but at greater distances from neighbors; fish with vision intact and the lateral line cut still held the shoal but at closer distances than normal.78 Vision sets the far limit, the lateral line sets the near limit.
Disable the whole lateral system and the behavior does not merely degrade, it fails. In antibiotic-treated fish the shoal radius went from about 15 cm to 25.97 plus or minus 4.27 cm, the proportion swimming in the same direction fell from 94.5 to 98 percent down to 63 percent, the distribution in the tank went from aggregative to random, and collisions appeared at about 3 per minute on days 4 to 9, sometimes hard enough to stun. Recovery tracked hair cell regrowth: shoaling reappeared between days 11 and 16, aggregative distribution by day 21, collisions stopped from day 25 and polarisation matched controls by day 29.6
That is a husbandry fact nobody in the hobby states. The lateral line is a surface organ of exposed hair cells, damaged by aminoglycoside antibiotics, by heavy metals and by handling. A tank treated with an aminoglycoside, or a shoal that has just been netted and bagged, may be behaviourally impaired for weeks after the fish look fine, and full sensory and behavioural recovery in the measured case took about a month6.
And the function that group living buys is measurable. Mosquitofish tested against a replica predator at group sizes of 1, 2, 4, 8 and 16 avoided it in 60 of 108 trials when solitary, 55.6 percent; groups of 8 and 16 were significantly better at P equals 0.02; the improvement matched a perfect many-eyes model; and the frequency of direction changes per fish fell as group size rose.9 Individuals in larger groups scanned less and the group still did better, because whoever spotted the threat first spread the information by positive feedback. That is what a shoal is for, and it is also why a predator-free glass box weakens the function without switching off the behavior.
Where the number came from
"A minimum of six" has no source. It is not a finding, and nobody has ever measured it.
This is a real contribution rather than a contrarian pose, and it is stated plainly because the alternative is to keep repeating a number whose origin nobody can produce.
The database line is a convention, not a measurement. The "in groups of 5 or more individuals" line appears identically for the cardinal tetra, the neon, the harlequin, the tiger barb and the gold barb, and traces to a single reference in aquarium hobby literature rather than to an experimental study10. The same figure for fish of very different sizes and behavior is the signature of a convention being copied.
The field's own journal never covered the subject. Of 1,228 indexed articles across 41 years, none has a title or keyword mentioning Corydoras, Betta, rasbora, danio, Otocinclus, goldfish, gourami, Apistogramma, hillstream loach or white cloud mountain minnow.11 Not one, in four decades.
The one attributable published number is twelve, and it is about looks. The Dutch style guide hosted by the Aquatic Gardeners Association says a dozen or so fish of one main species is the minimum and that single fish of many species fail to read as a school.12 It is honest about being a composition rule. It is not a welfare threshold and does not claim to be.
So six came from somewhere else, and no source we have found locates it. Forums, shop advice and aquarium atlases, repeated until it acquired the grammar of a finding.
What the experimental evidence actually says, which is a different shape
There are two studies that ask the hobby's question directly, and the answer is species-specific. A 2010 study tested group size in four species commonly kept in home aquaria and measured behavior and welfare indicators; a companion 2011 study did the same for mixed species assemblages. In both, the effects depend on which species.1314 That is close to the entire published evidence base behind community tank stocking advice. Two papers. The 2010 paper is paywalled with no abstract published in Crossref, PubMed, Europe PMC or Semantic Scholar, and its four species are inferred from its reference list rather than read.
The density evidence points the opposite way to the hobby's instinct. Zebrafish held for nine weeks at 1, 4, 8, 12 and 16 fish per liter showed higher free water cortisol per fish, altered vertical distribution and more frequent aggression at 1 fish per liter than at any higher density, and the authors conclude they can be held at 4 to 16 fish per liter without compromising welfare.15 For a shoaling species, too few is the measured welfare problem, and the tolerated range above it is very wide.
The real effects that have been measured sit at the very bottom of the range. Zebrafish in groups of four displayed stronger rheotaxis than zebrafish in pairs, and individuals in the larger group spent less time in the energetically costly leading position.16 Two against four. And preferred group size in fish is context dependent, shifting with circumstances rather than sitting at a species constant17, which is exactly what a fixed threshold cannot be.
So the defensible sentence is not "keep six or they will be stressed". It is: these are social animals, more of one species is better than a few of several, the evidence for a benefit is real but species-specific, the measured harms appear at the extreme low end rather than at a threshold, and nobody has established a number for the species in front of you. A group of twelve of one species is a better tank than two each of six species, and the reason is composition, physiology and the many-eyes effect together, not a rule.
The whole thing
One system, and the fish are the smallest part of it.
Read end to end, the material on this site describes one thing rather than many, and it is worth stating the shape of it plainly.
Carbon comes in and gets eaten. As leaf litter, as driftwood, as fish food. Bacteria and fungi break it down, animals eat the microbes rather than the substrate, and where oxygen runs out the same carbon leaves as methane instead of carbon dioxide.
Nitrogen is contested, not queued. Ammonium is wanted simultaneously by plants, by algae and by a specialist minority of nitrifiers that are a few percent of the filter. Who gets it depends on planting, light and how fast the ammonium appears.
Oxygen draws the map. Diffusion through water is ten thousand times slower than through air, so the substrate is a mosaic of oxic skins and anoxic pockets millimeters apart, and roots redraw it by pumping oxygen down.
Plants and microbes are one metabolism. The root releases oxygen, the bacteria respire and release carbon dioxide, the root takes it back. At the scale of two millimeters there is no boundary worth defending.
The community assembles in an order. Bacteria, then flagellates, then ciliates, then the larger animals, each waiting for the one below. Cycling is a succession, and the test kit reports on one guild in it.
The solid phase carries the life. Sediment, mulm and mature media hold the resting stages and the founding populations. Water carries much less. This was shown for protozoa in 1912, for bacteria in 2014 and for animals in 2023.
Only then, the animals. They are the thinnest layer and the one with the most confident folklore attached. A fish is a social, sensory animal with a lateral line and a preference for company, and none of that is quantified by the number six.
And all of it runs outdoors too. Lake Sevan is the same trophic cascade, the same detritivore chain and the same egg bank at a scale where the mistakes are permanent.
- McKnight, M.M., Szabolcs, N., Graham, A. and Neufeld, J.D. (2025). Microbial community succession of home aquarium biofilters associated with early establishment of comammox Nitrospira. ISME Communications 5(1): ycaf212. Open access, PMC12704419. The 1.6, 3.1 and 3.8 percent sponge figures and the 0.6 to 14.1 percent bead range, the dominant heterotrophic genera, the twelve week succession, and heterotrophs assisting nitrifier establishment. DOI 10.1093/ismeco/ycaf212 open access
- Pitcher, T.J. (1983). Heuristic definitions of fish shoaling behaviour. Animal Behaviour 31(2): 611 to 613. Paywalled, not read here. The origin of the shoal and school definitions. DOI 10.1016/S0003-3472(83)80087-6
- Pitcher, T.J. and Parrish, J.K. (1993). Functions of shoaling behaviour in teleosts. In: Pitcher, T.J. (ed.) Behaviour of Teleost Fishes, 2nd edition. London: Chapman and Hall, 363 to 439. Paywalled, not read here. DOI 10.1007/978-1-4684-8261-4_12
- Miller, N. and Gerlai, R. (2012). From schooling to shoaling: patterns of collective motion in zebrafish. PLoS ONE 7(11): e48865. Open access, PMC3498229. The verbatim source for the quoted Pitcher definition. DOI 10.1371/journal.pone.0048865 open access
- Ogi, A., Licitra, R., Naef, V., Marchese, M., Fronte, B., Gazzano, A. and Santorelli, F.M. (2020). Social preference tests in zebrafish: a systematic review. Frontiers in Veterinary Science 7: 590057. Open access, PMC7862119. An independent second reproduction of the same quoted phrase. DOI 10.3389/fvets.2020.590057 open access
- Faucher, K., Parmentier, E., Becco, C., Vandewalle, N. and Vandewalle, P. (2010). Fish lateral system is required for accurate control of shoaling behaviour. Animal Behaviour 79(3): 679 to 687. Author version open at the University of Liege repository ORBi, handle 2268/31748. The paraphrase of Pitcher 1983 and the statement that all schools are shoals with no clear dichotomy; also the antibiotic ablation experiment, giving the shoal radius of 25.97 plus or minus 4.27 cm against about 15 cm in controls, polarisation falling from 94.5 to 98 percent to 63 percent, the shift from aggregative to random distribution, collisions at about 3 per minute on days 4 to 9, and the recovery timeline through days 11 to 29 tracked against hair cell regrowth verified by scanning electron microscopy. DOI 10.1016/j.anbehav.2009.12.020 open access, author version
- Partridge, B.L. and Pitcher, T.J. (1980). The sensory basis of fish schools: relative roles of lateral line and vision. Journal of Comparative Physiology A 135: 315 to 325. Paywalled; read through the detailed account in source 46. Blinded fish holding position at greater distances, lateral-line-cut fish at closer distances. DOI 10.1007/BF00657647
- Partridge, B.L., Pitcher, T.J., Cullen, J.M. and Wilson, J. (1980). The three-dimensional structure of fish schools. Behavioral Ecology and Sociobiology 6(4): 277 to 288. Paywalled; read through the account in source 46. Cited alongside source 47 for the nearest-neighbour distance findings in blinded and lateral-line-ablated fish. DOI 10.1007/BF00292770
- Ward, A.J.W., Herbert-Read, J.E., Sumpter, D.J.T. and Krause, J. (2011). Fast and accurate decisions through collective vigilance in fish shoals. PNAS 108(6): 2312 to 2315. Open access, PMC3038776. The mosquitofish replica predator trials at group sizes 1, 2, 4, 8 and 16, the 60 of 108 solitary avoidances, the perfect many-eyes fit, and the fall in per-fish direction changes with group size. DOI 10.1073/pnas.1007102108 open access
- Froese, R. and Pauly, D. (eds.), FishBase, version 2025-2. The identical "in groups of 5 or more individuals" line across the cardinal tetra, neon tetra, harlequin rasbora, tiger barb and gold barb, and its single underlying reference, number 51539, which is aquarium hobby literature rather than an experimental study. Read species by species. fishbase.se
- The Aquatic Gardener complete index, 1985 to 2026. Published in full and free to read: 163 issues and 1,228 article entries. The source for the searches that return nothing on every standard aquarium fish. aquatic-gardeners.org
- Kutty, V. (2021). Some Rules for Dutch Style Aquascaping. Aquatic Gardeners Association, 15 pages. The only attributable published shoal-size number in aquarium keeping, and it is honest about being a composition rule. PDF
- Saxby, A., Adams, L., Snellgrove, D., Wilson, R.W. and Sloman, K.A. (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(3 to 4): 195 to 205. Paywalled, and no abstract is published in Crossref, PubMed, Europe PMC or Semantic Scholar. The only located study asking the hobby's question directly. DOI 10.1016/j.applanim.2010.04.008
- Sloman, K.A., Baldwin, L., McMahon, S. and Snellgrove, D. (2011). The effects of mixed-species assemblage on the behaviour and welfare of fish held in home aquaria. Applied Animal Behaviour Science 135(1 to 2): 160 to 168. Paywalled. The companion paper. DOI 10.1016/j.applanim.2011.08.008
- Andersson, M., Roques, J.A.C., Aliti, G.M., Ademar, K., Sundh, H., Sundell, K., Ericson, M. and Kettunen, P. (2022). Low holding densities increase stress response and aggression in zebrafish. Biology 11: 725. Open access, PMC9139139. The nine week trial at 1, 4, 8, 12 and 16 fish per litre, the elevated free water cortisol and aggression at 1 fish per litre, and the 4 to 16 conclusion. DOI 10.3390/biology11050725 open access
- Suriyampola, P.S., Iruri-Tucker, A.A., Padilla-Velez, L., Enriquez, A., Shelton, D.S. and Martins, E.P. (2022). Small increases in group size improve small shoals' response to water flow in zebrafish. Journal of Zoology 316: 271 to 281. Open access, PMC9269864. The measured difference between pairs and groups of four. DOI 10.1111/jzo.12952 open access
- Hoare, D.J., Couzin, I.D., Godin, J.-G.J. and Krause, J. (2004). Context-dependent group size choice in fish. Animal Behaviour 67: 155 to 164. Paywalled. The title is the finding. DOI 10.1016/j.anbehav.2003.04.004
Continue
Where to go next.
Nitrification, decomposition and roots
Who actually oxidises the ammonia, why tannins brake decomposition, and why root uptake substitutes for leaf uptake.
Succession, the egg bank and Lake Sevan
Succession, the egg bank, algae as a competitor, and Lake Sevan at whole lake scale.
Chemistry
The same system from the chemistry side, starting with the carbonate equilibrium.
Cycling and the bacteria
The practical version: how a filter actually starts, and the paper that named the organisms.
What is in the mud
The same processes in two litres of pond mud, where they run in weeks rather than decades.