How many fish in a 60, 100 or 200 litre tank (16, 26, 53 gal)
Updated on August 26, 2026 · ~6 min read
As a starting point, in freshwater you count about 1.5 litres for every centimetre of adult fish length — which is the old one-inch-per-gallon rule: 60 litres holds ~40 cm of fish, 100 litres ~67 cm, 200 litres ~133 cm. Marine tanks need far more room: 4 litres per cm fish-only, 5 in a reef.
"How many fish fit?" is the question with no exact answer, and the one where an honest estimate is worth much more than a cautious silence. This page explains the model we use, its numbers and — above all — where it does not work: a calculation whose limits you know is used better than one that claims to be exact.
The model, in one formula
You estimate capacity in centimetres of fish, starting from the adult size of the animals and not from the size of the specimen in the shop:
capacity (cm) = total litres of water / litres per cm
load (cm) = sum of (quantity × adult size × weight of the type)
stocking % = load / capacity × 100Litres per centimetre, by type of tank
| Type of tank | Litres per cm |
|---|---|
| Freshwater, planted included | 1.5 |
| Brackish | 2.0 |
| Fish-only marine | 4.0 |
| Reef | 5.0 |
The 1.5 for freshwater is the descendant of the old rule of thumb "one inch of fish per gallon" (2.54 cm per 3.785 litres) — and the conversion is almost exact, so if you think in gallons and inches, that is the same figure. Marine tanks are far sparser because salt water holds less oxygen, marine fish are more demanding and corals tolerate fewer nutrients.
How much each type of inhabitant weighs
| Type | Weight |
|---|---|
| Fish | 1.0 |
| Invertebrates (shrimp, snails, hermits) | 0.3 |
| Corals and plants | 0 |
Corals and plants produce no significant organic load — plants reduce it — so they do not enter the sum. Invertebrates weigh less than a fish of the same size.
The bands
| Stocking | How to read it |
|---|---|
| up to 85% | there is room |
| 85-110% | near the limit: regular maintenance is non-negotiable |
| over 110% | overstocked |
The tables: what your tank holds
Freshwater (1.5 L per cm):
| Volume | Capacity | 85% (comfortable limit) |
|---|---|---|
| 30 L (8 gal) | 20 cm (8 in) | 17 cm (7 in) |
| 60 L (16 gal) | 40 cm (16 in) | 34 cm (13 in) |
| 100 L (26 gal) | 67 cm (26 in) | 57 cm (22 in) |
| 150 L (40 gal) | 100 cm (39 in) | 85 cm (33 in) |
| 200 L (53 gal) | 133 cm (52 in) | 113 cm (44 in) |
| 300 L (79 gal) | 200 cm (79 in) | 170 cm (67 in) |
Marine (4 L per cm fish-only, 5 L per cm reef):
| Volume | Fish-only | Reef |
|---|---|---|
| 100 L (26 gal) | 25 cm (10 in) | 20 cm (8 in) |
| 200 L (53 gal) | 50 cm (20 in) | 40 cm (16 in) |
| 300 L (79 gal) | 75 cm (30 in) | 60 cm (24 in) |
| 500 L (132 gal) | 125 cm (49 in) | 100 cm (39 in) |
⚠️ The volume counts the real litres of water, not the ones printed on the box: substrate, rock and wood take up 10-15% of it, while a sump adds to it.
Three worked examples
60 litres (16 gal) of freshwater (capacity 40 cm). Ten adult neon tetras at 3 cm each = 30 cm, that is 75%: there is room. Adding six platies (5 cm) takes it to 60 cm, that is 150%: overstocked. A 60-litre tank holds one shoal, not two.
100 litres (26 gal) of freshwater (capacity 67 cm). Ten neons (30 cm) plus six bronze Corydoras (7 cm each, 42 cm) make 72 cm: 108%, near the limit. Dropping two Corydoras brings it to 87%.
200 litres (53 gal) reef (capacity 40 cm). Two clownfish (8 cm) = 16 cm, that is 40%. Adding a tang that reaches 20 cm takes it to 36 cm, that is 90%: near the limit with three fish. That is the sum which explains why a reef always looks "empty" compared with a freshwater tank.
Where the model is wrong, and it needs saying
It is a heuristic estimate, not a law. Length is a good indicator of mass and of waste production, but there are four things the sum does not see.
The shape of the animal. A 25 cm goldfish and a 25 cm eel do not produce the same load: the first weighs ten times as much. With stocky, greedy fish — goldfish first among them — the estimate is too generous and should be read conservatively.
Behaviour. A territorial fish can make a tank at "40%" unliveable, while a shoal of twenty small cyprinids gets along fine at 90%. Biological space and social space are two different things.
The shape of the tank. 100 litres long and low is worth more than 100 litres tall and narrow: the exchange surface is larger and there is more room to swim. And fast fish want length, not litres.
Filtration, food and maintenance. A tank at 100% with weekly changes and a generous filter is better off than one at 70% that gets neglected. The model describes an average case with regular maintenance.
The species' minimum volume always beats the sum
The sum says how much load the tank can carry; it does not say whether a species fits. These are two different constraints and the tighter of the two applies.
An oscar reaches 35 cm (14 in): in a 100-litre tank it would be 52% of the load, which looks comfortable, but the minimum volume for that species is 300 litres (79 gal) — it simply does not fit. The same for a clown loach (30 cm, minimum 300 L) or a Siamese algae eater (14 cm, minimum 100 L).
And there is the opposite constraint, which the load does not see at all: shoaling species. Neons, Corydoras, rasboras and most small cyprinids must be kept in groups of at least 6-8 individuals; three neons in a big tank are three stressed fish, even with the load at 20%.
The load is not the main criterion, it is the third. First you check that the species fits that volume, then that they can live together, then the load. A perfect sum on the wrong animals is still a tank that does not work.
How to get under, if you are over
There is not much to invent: fewer animals, or more litres. The half-measures that genuinely work are two — more frequent water changes and less food — and they exist to make a moderate overload sustainable, not to legitimise it. The objective signal is the nitrate reading before a change: if in freshwater it climbs week after week despite the routine, the tank is past its limit.
How Gurglee does it
Gurglee does this sum for you: on the Inhabitants screen it shows the stocking percentage and the band (room to spare / near the limit / overstocked), reading adult size from the species catalogue and the total volume of the tank (sump included), with the model on this page and its estimate disclaimer. Entries with no known size are flagged and not counted, instead of quietly disappearing. And before you add an inhabitant the compatibility check warns you if the species does not fit that volume or will not live with what is already there — the constraint which, as said above, beats the load calculation.
Related guides
Freshwater fish compatibility
How to decide whether two freshwater fish can live together: water, volume, temperament and size, with the four checks to run before buying.
Why new fish die soon after you buy them
New fish die in the first days: the real causes in order of frequency, how to acclimatise a fish properly and what to do before buying any more.
High nitrate: how to lower it for real
Why nitrate climbs in an aquarium, which values to hold in freshwater and in a reef, and the methods that actually lower it — from water changes to plants.
Aquarium water change: how much, how often, how
How much water to change and how often, how to prepare the new water and the steps to do it without stressing the fish. With the arithmetic of the effect on nitrate.