Aquarium stocking density reference table
Short answer: stocking a tank well depends on a fish's waste output (bioload) and behavior — not its length, and not the “1 inch per gallon” rule. The table below gives, for 30 popular freshwater species, a conservative minimum tank size, minimum group size, adult size, relative bioload, and temperament. Use it to build a balanced stocking list: pick species whose minimum tank size your tank meets, honor each schooling fish's minimum group, and keep total bioload comfortably below what your filter and weekly water changes can hold.
The reference table
Values are conservative starting points drawn from mainstream freshwater fishkeeping consensus. Tank sizes are US gallons. “Min group” is the smallest number that keeps a schooling or social species healthy — for true schoolers, more is better. Always confirm the needs of the exact species you intend to keep before you buy.
| Species | Category | Adult size (in) | Min tank (gal) | Min group | Bioload | Temperament |
|---|---|---|---|---|---|---|
| Neon tetra | Small schooler | 1.5 | 10 | 6 | Low | Peaceful |
| Cardinal tetra | Small schooler | 1.5 | 15 | 6 | Low | Peaceful |
| Ember tetra | Nano schooler | 0.8 | 10 | 6 | Very low | Peaceful |
| Harlequin rasbora | Small schooler | 1.75 | 15 | 6 | Low | Peaceful |
| Chili rasbora | Nano schooler | 0.7 | 5 | 8 | Very low | Peaceful, shy |
| White Cloud Mountain minnow | Small schooler | 1.5 | 10 | 6 | Low | Peaceful (cool water) |
| Zebra danio | Small schooler | 2 | 10 | 6 | Medium | Active, can fin-nip |
| Celestial pearl danio | Nano schooler | 1 | 10 | 6 | Very low | Peaceful, shy |
| Guppy | Livebearer | 2 | 10 | 3 | Medium | Peaceful, prolific |
| Endler's livebearer | Livebearer | 1 | 10 | 3 | Low | Peaceful, prolific |
| Platy | Livebearer | 2.5 | 15 | 3 | Medium | Peaceful |
| Molly | Livebearer | 4 | 30 | 3 | High | Peaceful, big for a livebearer |
| Swordtail | Livebearer | 5 | 29 | 3 | Medium | Active; males can spar |
| Betta (male) | Centerpiece | 2.75 | 5 | 1 | Low | Solitary; no fin-nippers |
| Honey gourami | Centerpiece | 2 | 10 | 1 | Low | Peaceful, shy |
| Dwarf gourami | Centerpiece | 3.5 | 15 | 1 | Medium | Generally peaceful |
| Pearl gourami | Centerpiece | 4.5 | 30 | 1 | Medium | Peaceful, needs length |
| Corydoras catfish | Bottom dweller | 2.5 | 20 | 6 | Low | Peaceful, social |
| Pygmy corydoras | Bottom dweller | 1 | 10 | 8 | Very low | Peaceful, shy |
| Kuhli loach | Bottom dweller | 3.5 | 20 | 6 | Low | Peaceful, hides |
| Bristlenose pleco | Bottom dweller | 4.5 | 25 | 1 | High | Peaceful, big waste |
| Otocinclus | Bottom dweller | 1.75 | 10 | 6 | Low | Peaceful; mature tank only |
| Cherry shrimp | Invertebrate | 1.25 | 5 | 6 | Very low | Peaceful; needs no predators |
| Amano shrimp | Invertebrate | 2 | 10 | 3 | Very low | Peaceful algae crew |
| Nerite snail | Invertebrate | 1 | 5 | 1 | Very low | Peaceful; won't overbreed |
| Mystery snail | Invertebrate | 2 | 5 | 1 | Medium | Peaceful; ~5 gal each |
| German blue ram | Dwarf cichlid | 2.5 | 20 | 2 | Medium | Peaceful; warm, stable water |
| Apistogramma | Dwarf cichlid | 3 | 20 | 2 | Medium | Territorial when breeding |
| Angelfish | Large community | 6 | 29 | 1 | High | Semi-aggressive; tall tank |
| Fancy goldfish | Coldwater | 7 | 20 | 1 | Very high | Cool water; +10 gal per extra fish |
Goldfish are the standout: a single fancy goldfish needs roughly a 20-gallon start and about 10 more gallons per additional fish, and its very high bioload is why goldfish should never share a tropical community tank.
How to read the columns
- Adult size — the length to plan for, not the inch-long juvenile in the store bag. Stocking for the adult is what keeps a tank from becoming overcrowded in six months.
- Minimum tank size — the smallest tank that gives the species enough water volume and floor length to behave normally. A long, active swimmer needs footprint, not just gallons.
- Minimum group size — for schooling and social fish, the floor below which they get stressed, hide, lose color, or turn nippy. For solitary or paired species it is 1 or 2.
- Bioload — relative waste output, from very low to very high. This is the real ceiling on how much you can keep, and it is what you balance with filtration and water changes.
- Temperament — the shorthand for who can live with whom. Mixing peaceful schoolers with a fin-nipper or a semi-aggressive cichlid is how good stocking lists go wrong.
Using the table to build a stocking list
A balanced freshwater community usually layers the tank: a mid-water school, a centerpiece, a bottom group, and cleanup invertebrates. Pick from each layer using species whose minimum tank size your tank meets, honor every schooler's minimum group, and keep your combined bioload modest — one high-waste fish plus several low-waste ones, not a tank full of heavy feeders.
| Layer | Example pick (20-gallon) | Why it works |
|---|---|---|
| Mid-water school | 12–15 neon or cardinal tetras | Low bioload, fills the open water, honors the min-6 school. |
| Centerpiece | 1 honey gourami or dwarf gourami | A single focal fish; peaceful with a tetra school. |
| Bottom group | 6 corydoras catfish | Social, low waste, works the substrate; needs a group. |
| Cleanup crew | A nerite snail or a cherry shrimp colony | Near-zero bioload, eats algae and leftovers. |
That list keeps every layer within a 20-gallon's limits and leaves headroom on bioload — the layered, slightly understocked approach that stays stable. For the size-by-size version of this logic, see how many fish fit in a 20-gallon tank, and let the stocking calculator total the load for your exact list.
Why bioload and behavior beat counting inches
The old “1 inch of fish per gallon” rule survives because it is easy, but it gets the two most important factors wrong:
- Waste is not linear with length. A 6-inch goldfish does not equal six 1-inch tetras — it produces vastly more waste, which is why goldfish sit at “very high” bioload and tetras at “low.” Stocking by inches ignores this entirely.
- It ignores minimum group and footprint. The inch rule will happily approve a single schooling fish, or an active swimmer in a short tank — both of which the table forbids through the minimum-group and minimum-tank columns.
This is why our stocking calculator and our how many fish per gallon, really guide both work from bioload and behavior rather than a single inch number. The table on this page is the quick-reference version of that same thinking.
The two tools that make stocking decisions easy: a liquid water-test kit to watch nitrate as you add fish, and right-sized filtration and heat for the tank.
Test & monitor: freshwater test kit · thermometer
Filtration & heat: our filter-size calculator · hang-on-back filter · our heater-wattage calculator · submersible heater
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Frequently asked questions
What is a good stocking density for a freshwater aquarium?
There is no single number for every tank, because stocking depends far more on bioload and behavior than on length. Start from each species' minimum tank size and minimum group size, keep total stocking comfortably below the point where your filter and water changes can no longer hold nitrate low, and understock rather than fill to the brim. The table above gives conservative minimum tank size, minimum group size, adult size, and bioload for 30 popular species so you can build a balanced list instead of guessing.
Is the 1-inch-per-gallon rule accurate?
No. It ignores the two things that actually limit stocking: bioload and behavior. A 3-inch goldfish produces far more waste than three 1-inch tetras, so equal inches do not mean equal load, and the rule has no concept of minimum school size or tank footprint. Use minimum tank size, minimum group size, and bioload instead — the way this table is organized.
How do I know if my aquarium is overstocked?
The clearest signal is water chemistry: if nitrate climbs quickly between weekly changes, or you ever detect ammonia or nitrite in a cycled tank, the bioload is outrunning your filtration and maintenance. Behavioral signs include constant surface gasping, fish that never use the open water, aggression in cramped quarters, and frequent disease. Overstocking is about waste and behavior, not just whether the fish physically fit.
What is bioload in an aquarium?
Bioload is the total waste your livestock produces, which your filter bacteria and water changes must process. It depends on body mass, metabolism, and feeding — not length — which is why a single fancy goldfish has a far higher bioload than a school of neon tetras of the same combined length. In the table, bioload is rated very low to very high so you can balance a heavy centerpiece against low-waste schoolers and invertebrates.
Related: Aquarium stocking calculator · How many fish per gallon, really · 5-gallon · 10-gallon · 20-gallon · Neon tetras in a 20