One question about every aquarium, asked six ways
How many fish in a ten gallon tank. In a twenty. In a five. It is the same question with the numbers swapped, and the answer people are given is almost always a rule of thumb: an inch of fish per gallon, or a number somebody counted in their own tank once. Our tetra guide shows what the first of those does to a real fish: by the inch rule six Congo tetras fit in eighteen gallons, and the source that names the species asks for thirty.
Not one source we compile from uses that rule. What they publish instead is two figures per species, a minimum tank and a group size, and those two figures answer the question directly for the animals we hold records on. The grid below is those figures, arranged by the size of tank you already own.
A minimum volume
The smallest tank the source will state for the species. It is a floor and not a target: our records give it as a range wherever the source did.
A group size
How many of the species belong together. For a shoaling fish this is not a preference, it is most of what the source is telling you.
One rule of ours
One group per layer. It is the only thing on this page that is not a recorded figure, and it is named as ours everywhere it changes an answer.
Every record, at every size
Find your tank down the left. A cell is the group the record asks for, or a refusal with the volume that record wants instead. Nothing here is scaled, averaged or estimated: it is the two fields, printed.
| Tank | African dwarf frog | Amano shrimp | Amazon sword | Angelfish | Anubias | Assassin snail | Axolotl | Betta | Black molly | Bristlenose pleco | Celestial pearl danio | Cherry shrimp | Chili rasbora | Chinese algae eater | Clown killifish | Clown loach | Common pleco | Corydoras | Damselfish | Discus | Dojo loach | Ember tetra | Flowerhorn | German blue ram | Ghost shrimp | Goldfish | Guppy | Honey gourami | Hornwort | Java fern | Java moss | Kuhli loach | Least killifish | Mandarin goby | Marimo moss ball | Nerite snail | Oscar fish | Otocinclus | Pea puffer | Platy | Rabbit snail | Ramshorn snail | Redtail catfish | Siamese algae eater | Sparkling gourami | Vampire shrimp | Water sprite | Water wisteria |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 5 gal | 2+from 5 gal per frog | noasks for 10 gal | not recordedno minimum stated | noasks for 55 gal | not recordedno minimum stated | noasks for 10 gal | noasks for 20 gal | Onefrom 5 to 10 gal | noasks for 20 gal | noasks for 20 gal | noasks for 10 gal | Colonyfrom 5 to 10 gal | 8 to 12from 5 gal | noasks for 50 gal | 8 to 10from 5 gal | noasks for 120 gal | noasks for 75 gal | noasks for 10 to 20 gal | noasks for 30 gal | noasks for 75 gal | noasks for 55 gal | noasks for 10 gal | noasks for 70 gal | noasks for 20 gal | 3 to 4 per gallonfrom 5 gal | noasks for 20 gal | noasks for 10 gal | noasks for 10 gal | not recordedno minimum stated | not recordedno minimum stated | not recordedno minimum stated | noasks for 20 gal | Six, or a trio, or a pairfrom 5 gal | noasks for 30 gal | not recordedno minimum stated | no group statedfrom 2 to 3 gal | noasks for 75 gal | noasks for 10 gal | 1 m : 2-3 ffrom 5 gal | noasks for 10 gal | noasks for 10 gal | Not set by youfrom 5 gal | noasks for 2,700 gal (10,368 L) | noasks for 40 gal | noasks for 10 gal | noasks for 20 gal | not recordedno minimum stated | not recordedno minimum stated |
| 10 gal | 2+from 5 gal per frog | 2 per gallonfrom 10 gal | not recordedno minimum stated | noasks for 55 gal | not recordedno minimum stated | 1 to 6from 10 gal | noasks for 20 gal | Onefrom 5 to 10 gal | noasks for 20 gal | noasks for 20 gal | Six, or twentyfrom 10 gal | Colonyfrom 5 to 10 gal | 8 to 12from 5 gal | noasks for 50 gal | 8 to 10from 5 gal | noasks for 120 gal | noasks for 75 gal | 6+from 10 to 20 gal | noasks for 30 gal | noasks for 75 gal | noasks for 55 gal | 8 to 10from 10 gal | noasks for 70 gal | noasks for 20 gal | 3 to 4 per gallonfrom 5 gal | noasks for 20 gal | Trio+from 10 gal | 4 to 6from 10 gal | not recordedno minimum stated | not recordedno minimum stated | not recordedno minimum stated | noasks for 20 gal | Six, or a trio, or a pairfrom 5 gal | noasks for 30 gal | not recordedno minimum stated | no group statedfrom 2 to 3 gal | noasks for 75 gal | 6+from 10 gal | 1 m : 2-3 ffrom 5 gal | 3 to 6from 10 gal | 3 or morefrom 10 gal | Not set by youfrom 5 gal | noasks for 2,700 gal (10,368 L) | noasks for 40 gal | Four or morefrom 10 gal | noasks for 20 gal | not recordedno minimum stated | not recordedno minimum stated |
| 20 gal | 2+from 5 gal per frog | 2 per gallonfrom 10 gal | not recordedno minimum stated | noasks for 55 gal | not recordedno minimum stated | 1 to 6from 10 gal | 1from 20 gal | Onefrom 5 to 10 gal | Four, one malefrom 20 gal | 1 malefrom 20 gal | Six, or twentyfrom 10 gal | Colonyfrom 5 to 10 gal | 8 to 12from 5 gal | noasks for 50 gal | 8 to 10from 5 gal | noasks for 120 gal | noasks for 75 gal | 6+from 10 to 20 gal | noasks for 30 gal | noasks for 75 gal | noasks for 55 gal | 8 to 10from 10 gal | noasks for 70 gal | A pairfrom 20 gal | 3 to 4 per gallonfrom 5 gal | No numberfrom 20 gal | Trio+from 10 gal | 4 to 6from 10 gal | not recordedno minimum stated | not recordedno minimum stated | not recordedno minimum stated | 5 to 6from 20 gal | Six, or a trio, or a pairfrom 5 gal | noasks for 30 gal | not recordedno minimum stated | no group statedfrom 2 to 3 gal | noasks for 75 gal | 6+from 10 gal | 1 m : 2-3 ffrom 5 gal | 3 to 6from 10 gal | 3 or morefrom 10 gal | Not set by youfrom 5 gal | noasks for 2,700 gal (10,368 L) | noasks for 40 gal | Four or morefrom 10 gal | Three or morefrom 20 gal | not recordedno minimum stated | not recordedno minimum stated |
| 29 gal | 2+from 5 gal per frog | 2 per gallonfrom 10 gal | not recordedno minimum stated | noasks for 55 gal | not recordedno minimum stated | 1 to 6from 10 gal | 1from 20 gal | Onefrom 5 to 10 gal | Four, one malefrom 20 gal | 1 malefrom 20 gal | Six, or twentyfrom 10 gal | Colonyfrom 5 to 10 gal | 8 to 12from 5 gal | noasks for 50 gal | 8 to 10from 5 gal | noasks for 120 gal | noasks for 75 gal | 6+from 10 to 20 gal | noasks for 30 gal | noasks for 75 gal | noasks for 55 gal | 8 to 10from 10 gal | noasks for 70 gal | A pairfrom 20 gal | 3 to 4 per gallonfrom 5 gal | No numberfrom 20 gal | Trio+from 10 gal | 4 to 6from 10 gal | not recordedno minimum stated | not recordedno minimum stated | not recordedno minimum stated | 5 to 6from 20 gal | Six, or a trio, or a pairfrom 5 gal | noasks for 30 gal | not recordedno minimum stated | no group statedfrom 2 to 3 gal | noasks for 75 gal | 6+from 10 gal | 1 m : 2-3 ffrom 5 gal | 3 to 6from 10 gal | 3 or morefrom 10 gal | Not set by youfrom 5 gal | noasks for 2,700 gal (10,368 L) | noasks for 40 gal | Four or morefrom 10 gal | Three or morefrom 20 gal | not recordedno minimum stated | not recordedno minimum stated |
| 40 gal | 2+from 5 gal per frog | 2 per gallonfrom 10 gal | not recordedno minimum stated | noasks for 55 gal | not recordedno minimum stated | 1 to 6from 10 gal | 1from 20 gal | Onefrom 5 to 10 gal | Four, one malefrom 20 gal | 1 malefrom 20 gal | Six, or twentyfrom 10 gal | Colonyfrom 5 to 10 gal | 8 to 12from 5 gal | noasks for 50 gal | 8 to 10from 5 gal | noasks for 120 gal | noasks for 75 gal | 6+from 10 to 20 gal | Onefrom 30 gal | noasks for 75 gal | noasks for 55 gal | 8 to 10from 10 gal | noasks for 70 gal | A pairfrom 20 gal | 3 to 4 per gallonfrom 5 gal | No numberfrom 20 gal | Trio+from 10 gal | 4 to 6from 10 gal | not recordedno minimum stated | not recordedno minimum stated | not recordedno minimum stated | 5 to 6from 20 gal | Six, or a trio, or a pairfrom 5 gal | Onefrom 30 gal | not recordedno minimum stated | no group statedfrom 2 to 3 gal | noasks for 75 gal | 6+from 10 gal | 1 m : 2-3 ffrom 5 gal | 3 to 6from 10 gal | 3 or morefrom 10 gal | Not set by youfrom 5 gal | noasks for 2,700 gal (10,368 L) | 5 to 6from 40 gal | Four or morefrom 10 gal | Three or morefrom 20 gal | not recordedno minimum stated | not recordedno minimum stated |
| 55 gal | 2+from 5 gal per frog | 2 per gallonfrom 10 gal | not recordedno minimum stated | 6 juvenilesfrom 55 gal | not recordedno minimum stated | 1 to 6from 10 gal | 1from 20 gal | Onefrom 5 to 10 gal | Four, one malefrom 20 gal | 1 malefrom 20 gal | Six, or twentyfrom 10 gal | Colonyfrom 5 to 10 gal | 8 to 12from 5 gal | Onefrom 50 gal | 8 to 10from 5 gal | noasks for 120 gal | noasks for 75 gal | 6+from 10 to 20 gal | Onefrom 30 gal | noasks for 75 gal | 3 or morefrom 55 gal | 8 to 10from 10 gal | noasks for 70 gal | A pairfrom 20 gal | 3 to 4 per gallonfrom 5 gal | No numberfrom 20 gal | Trio+from 10 gal | 4 to 6from 10 gal | not recordedno minimum stated | not recordedno minimum stated | not recordedno minimum stated | 5 to 6from 20 gal | Six, or a trio, or a pairfrom 5 gal | Onefrom 30 gal | not recordedno minimum stated | no group statedfrom 2 to 3 gal | noasks for 75 gal | 6+from 10 gal | 1 m : 2-3 ffrom 5 gal | 3 to 6from 10 gal | 3 or morefrom 10 gal | Not set by youfrom 5 gal | noasks for 2,700 gal (10,368 L) | 5 to 6from 40 gal | Four or morefrom 10 gal | Three or morefrom 20 gal | not recordedno minimum stated | not recordedno minimum stated |
What the grid shows that nobody writes down
Ten gallons fills four of the five layers this site can fill. Below it, at five, three are available, because the corydoras and the ember tetra both ask for ten. Above it the count moves once: the goldfish occupies a layer of its own, all levels, and it asks for twenty, so the fifth layer opens at twenty and not before.
So the honest answer to what does a bigger tank buy me is not more species, at least not from the records here. It buys the groups room, it buys the water stability, and it buys margin for the day something goes wrong. Those are worth paying for. A fifth layer is not on offer.
The five gallon question
Ten records state a minimum at or under five gallons: the nerite snail at two to three, the pea puffer, the ghost shrimp, the chili rasbora, the ramshorn snail, the clown killifish and the least killifish at five, the african dwarf frog at five per frog, and the betta and the cherry shrimp at five to ten. That is three surface fish, a mid water predator, a school, an amphibian and four substrate grazers, and six of the fifty one refusals on this site are inside that group of ten.
Which leaves a five gallon tank with a real answer and a small one. It is not a failure of the tank. It is what our sources will state, and the alternative is a page that fills the gap with a fish nobody has verified for you.
The stocking calculator runs the same figures for one volume at a time and adds the heater setting the build would need.
Where the group sizes come from, on every care record
Twenty eight of our forty eight records carry a group figure from a source, and nine do not. Five of the nine are plants, which do not have one. Of the other three, the goldfish record deliberately states none, because the only source that gives a number gives it in order to reject it, the nerite snail record has no figure at all, and the oscar record states one or a pair as our reading rather than as any source’s, because none of its six sources gives a number. The angelfish figure is the odd one in the list: six juveniles, bought not because the fish needs a school but because a pair cannot be picked out at the shop. They are worth reading as sentences and not as numbers in a table.
The higher of two source figures, taken because the smaller number was a floor and this one was what the source called comfortable. Eight fish is 80 percent of a ten gallon tank’s stocking in one species, which is the point.
Of one species is the part people drop. Six mixed corydoras is not a group of six, it is several groups of one or two, and the source is specific about that.
Minimum, and in a tank the record already puts at twenty gallons. This is the record that makes the bottom layer expensive, and it is the reason a small tank has fewer honest options than a catalog suggests.
The one group figure on this site that is a hard number rather than a range. The cherry shrimp entry is the opposite case: a colony, which is a way of saying the count is not the useful field.
What this grid does not count
Four honest gaps, because a stocking answer that hides its assumptions is worse than no answer.
Every figure here assumes a filtered, cycled tank, which is what the sources assume. None of them states a stocking number as a function of filter capacity, so neither do we.
Two tanks with the same fish and different feeding are different tanks. That is real and it is not a number anyone publishes, so it cannot be in the grid.
Our records name cover as husbandry, cholla wood and moss for the shrimp, buried time for the kuhli loach. None of them converts it into a stocking allowance.
Neon tetras, platies and mollies are the fish most people are still asking about, and none of them has a record here yet. The grid holds forty eight animals because forty eight are verified.
Questions about aquarium stocking
How many fish in a 10 gallon tank?
From our records, one group per layer: a betta at the surface, eight to ten ember tetras or a guppy trio in mid water, six or more corydoras on the bottom, and on the substrate a shrimp colony, a nerite snail or six otocinclus. That comes to twenty or more animals, and it is the largest build this site can support in that volume.
How many fish in a 5 gallon tank?
Two layers, not four. The records that state a minimum at or under five gallons are the nerite snail at two gallons and the betta and cherry shrimp at five to ten, and the betta with shrimp pairing is refused on our records. So it is one surface animal and one grazer, or a shrimp colony and a snail.
How many fish in a 20 gallon tank?
The same four layers as a ten gallon, with the kuhli loach now available on the bottom at five to six. Twenty gallons does not add a fifth layer to this site, it adds a species option and a great deal of room for the groups already there.
Is the inch per gallon rule any good?
Not one of our sources uses it, which is the most useful thing we can say about it. It also gets our own records wrong: the kuhli loach is 3.5 to 3.9 inches and asks for twenty gallons, while eight ember tetras at 0.8 inches each fit in ten.
Why does a bigger tank not mean more species?
Because our rule is one group per layer and there are four layers. Once ten gallons has filled all four, more volume goes to the groups rather than to a fifth kind of animal. That is a limit of this site’s forty eight records rather than of your tank.
How many guppies in a 10 gallon tank?
The record is written now, and it answers the question by refusing to give a number of fish. It states ten gallons and a ratio, one male to two or three females, and then the figure that actually decides it: between 5 and 100 young from a single female, every four to six weeks, for six months after one mating. The starting count is not the constraint.
Can I keep one fish on its own?
For the betta our source states one male and never two, so a single fish is the recorded arrangement. For the corydoras, the kuhli loach and the ember tetra the source states a group, and keeping one is not a smaller version of the record, it is a different thing from what the source described.
What is a layer?
Where the record says the animal spends its time: surface, mid water, bottom, or on the substrate. It is a field on every record, and it is the field our one rule runs on, because two groups in the same water are in each other’s way in a way that two groups in different water are not.
Does the nerite snail count towards stocking?
On our grid it occupies the substrate layer and it states a two gallon minimum, which is the smallest figure on the site. What it does not state is a group size, so the cell says so. No source we hold gives a number of snails per gallon.
Where do these numbers come from?
Two fields on our own records, the minimum tank and the group size, each of which names the source it came from on the record page. The grid does no arithmetic on them beyond comparing your volume against the minimum.
What is not here yet
Three guides are filed under this topic, two about a group of species and one about a group of one species: our tetra fish guide, our rasboras guide and our female betta fish guide. The questions waiting to be written are specific and countable: how many guppies in a ten gallon, how many neon tetras in a ten, how many fish in a five, how many in a twenty. Every one of them needs a record on the species first, because the answer is that record’s two fields and nothing else.
That is the order this site works in, and it is slower than writing the answer straight out. It also means that when a guide appears here, the number in it will already be printed in the grid above, from the same field, on the same day.
Bioload and filtration, and why this grid has neither
Every other aquarium stocking resource you will find answers this question with a percentage. You enter a tank and a filter, it weighs the rated capacity against the adult size and waste output of each fish, and a bar fills up. This grid returns no such number, and the reason is not that the idea is wrong.
It is that none of the care sources we compile from publishes a waste figure for a single one of these species. Not one. A bioload percentage built on this site's records would be arithmetic performed on estimates we invented, and inventing the input is the one thing this whole project exists not to do.
- Bioload: refused. No source, no figure, no bar. If you arrived from a tool that told you a 10 gallon aquarium was 68 percent stocked, this page cannot reproduce that number and is not trying to.
- Filtration: not recorded as a number. No filter rating and no turnover multiple appears on any record, because our sources attach none. Where filtration appears it appears in words, such as the gentle flow the kuhli loach record describes and the slow flow with a sponge filter another record attributes to Aquarium Co-Op by name.
- Compatibility: modeled, the hard way. Not as a score out of a hundred but as a list of refusals, each traceable to a source that says never, and blank where no source ruled anything. A grid that scores compatibility is telling you how confident it is.
- Which leaves three fields doing the work: the minimum tank, the group size and the water column. Two come straight off the records and the third is the only thing on this page we derive.
What a community tank means here
Read the grid across rather than down and the community question answers itself. At five gallons the records support one occupied layer, so a community of one. At ten a community of two becomes possible, a surface fish and one group. Above twenty the volume stops constraining anything and the refusals take over, and refusals do not improve with gallons.
So a community aquarium in the ordinary sense begins where the records stop objecting rather than where the water runs out. The tank mates topic holds the objections, and the stocking calculator runs the same three fields for one volume at a time and prints the heater setting the build needs.
The rule this page will not print
One inch of fish per gallon is published by one source we compile from, PetMD, on a page written by a veterinarian, and this page still will not use it. It ignores floor space, it ignores the layer an animal occupies, and it produces the same answer for a betta and a goldfish. It is the most repeated stocking rule in the hobby and there is no record on this site that supports it, which is why the grid prints group sizes instead.
Every figure in the grid is a field off a care record, and every record names the source it came from. That is a narrower promise than a percentage and it is one this page can keep.
Guides on this topic
A guide is where a stocking question that is not a single species gets answered, and all three of the ones here are about how many rather than about which. The first is about the group of fish this page argues with most, because the rule this page refuses is published on a tetra care sheet. The rasboras guide is here for the opposite reason: two published lists give the same fish groups of 8 to 12 and of 20 or more, so the question that page answers is which number a keeper is entitled to trust.