How this is calculated
Every species in the database carries a bioload factor — a relative measure of how much ammonia one adult produces. One unit is one adult bronze corydoras. A neon tetra is 0.7 units; an adult oscar is 8; a common pleco is 12. The factor scales roughly with body mass and appetite rather than with length, which is exactly where the inch-per-gallon rule falls apart.
Your stocking total is then measured against four separate capacities. The tool reports the highest of the four ratios as your stocking percentage, and names which one it was, because that is the one you would have to fix.
- Water volume. Net gallons after substrate and decor displacement, at 0.62 bioload units per gallon. A planted tank gets 15% more, because growing plants consume ammonia and nitrate directly.
- Filtration turnover. A stocked community tank wants roughly six tank volumes an hour through the filter. Your rated gallons per hour divided by six gives the volume your filter can actually service; if that is less than your tank, filtration is the limit. Box ratings are measured with no media in the filter, so treat them as optimistic.
- Surface area. Oxygen enters and carbon dioxide leaves at the water surface, and that surface is length × width. Height adds volume but no gas exchange, which is why a tall column tank holds fewer fish than its gallons suggest. Budgeted at one bioload unit per 20 square inches, which is about 60% more permissive than the old one-inch-per-twelve- square-inches rule because a modern filter return agitates the surface. If you also run an air stone or a spray bar, you have more headroom here than this shows.
- Footprint and swimming room. Two checks, whichever is worse: whether any single species needs a longer tank than you have, and total adult body inches against 2.5 inches of fish per inch of tank length. A twelve-inch fish in a four-foot tank fails on footprint long before it fails on gallons.
Worked example — the tank this page loads with. A 48 × 13 × 21 inch tank is 13,104 cubic inches, or 56.7 gallons. Take 10% for substrate and decor and the net volume is 51.0 gallons — already less than the "55 gallon" on the box.
Twelve neon tetras (12 × 0.7 = 8.4), eight bronze corys (8 × 1.0 = 8.0), two angelfish (2 × 3.0 = 6.0), four otocinclus (4 × 0.5 = 2.0) and one bristlenose pleco (3.0) come to 27.4 bioload units.
Volume capacity: 51.0 × 0.62 × 1.15 for planted = 36.4 units, so 75%. Filtration: 400 gph ÷ 6 = 66.7 gallons serviced × 0.62 = 41.3 units, so 66%. Surface: 48 × 13 = 624 square inches ÷ 20 = 31.2 units, so 88%. Footprint: the angelfish and the bristlenose both want 30 inches of length and you have 48, so 63%.
Surface area binds first. The tank is 88% stocked, and the thing standing between you and more fish is gas exchange, not gallons — which no inch-per-gallon rule would ever tell you.
How compatibility is decided
Every pair of species in your list is run through the same seven checks, and the worst result is what the matrix shows. Where two species trigger several problems, all of them are listed underneath.
- Predation by mouth size. Each predatory species carries the largest tankmate it will swallow. If that figure reaches a tankmate's adult length it is a cross, not a caution — and it is scored on adult sizes, because the fish you bought at an inch is not the fish that will be in the tank in a year.
- Fin nipping. A species flagged as a nipper against a species flagged as long-finned is a cross. This is the most common avoidable failure in a community tank and it is entirely predictable.
- Temperament. Aggressive against peaceful is a cross. Aggressive against semi-aggressive, or a semi-aggressive fish more than three and a half times the length of a peaceful one, is a caution.
- Temperature overlap. No shared range is a cross. Under 3°F of shared range is a caution, because one of the two is permanently at the edge of its tolerance.
- pH overlap. Ranges that do not meet are a caution, or a cross if they miss by more than 0.8 pH. Commercially bred stock adapts further than published ranges suggest — the conflicts that matter are between the extremes.
- Hardness overlap. Same logic, crossing at more than 6 dGH apart. Hardness is the parameter most often ignored and it is the one that quietly shortens lives.
- Invertebrates and floor territory. Fish that hunt shrimp or snails are flagged against them, and two species that both defend the floor against similar shapes are flagged against each other.
What this does not account for
- Individual temperament. Some bettas live peacefully in a community for years; some cannot share a tank with anything. A database cannot know which one you bought.
- Layout. Broken sight lines, caves, dense planting and driftwood change aggression outcomes more than almost anything else, and none of it is in these numbers.
- Wild versus farmed stock. Wild-caught fish hold much tighter to their published parameters. Farmed fish, several generations into hard tap water, often do not.
- Sumps and refugiums. Extra water volume in a sump is real volume, but no extra surface area unless it is open to air. Add sump gallons to the tank dimensions at your own judgement.
- Surface agitation and altitude. Air stones, spray bars and wave makers raise the oxygen ceiling; high altitude and high temperature lower it. Water holds meaningfully less oxygen at 84°F than at 74°F.
- Feeding. Bioload is a function of what goes in. Overfeeding a lightly stocked tank produces more nitrate than careful feeding of a full one.
- Disease and quarantine. None of this substitutes for quarantining new fish.
- Breeding. A peaceful cichlid becomes an aggressive one the day it spawns, and livebearers will fill the tank whether or not you planned the bioload for it.
- Your actual water. A utility report is an annual average across a network. Test your own tap, and test it again after it has sat out overnight.
Common questions
Why is one inch per gallon wrong?
Waste output rises far faster than length. A six-inch goldfish produces vastly more ammonia than six one-inch tetras, and the rule ignores filtration, footprint, surface area, adult size and whether the fish can live together at all. It survives because it is easy to remember, not because it works. There is a longer version of this argument here.
What does bioload actually mean?
The rate at which the fish produce ammonia, which the filter bacteria convert to nitrite and then nitrate, which you remove with water changes. It scales with body mass and feeding rather than with length, so one adult pleco can outweigh an entire school of tetras.
How much water should I change each week?
Enough to stop nitrate climbing between changes. Lightly stocked and planted can be 15–20% weekly; heavily stocked is 40–50%, often split into two. Plate V gives you a figure derived from your actual stocking, but the honest answer comes from testing nitrate before and after a change for a few weeks and adjusting.
Do I really need six of a schooling fish?
Yes, and usually more. Shoaling species kept in twos and threes are chronically stressed: they hide, feed poorly, lose colour and turn on each other. Six is the floor. Most of these species look like a different fish entirely at ten or twelve.
Can I keep an angelfish with neon tetras?
Not permanently. A juvenile angelfish ignores them; an adult has a mouth big enough to take a neon and will eventually use it, usually within a month of maturing. Choose a deeper-bodied tetra an adult cannot swallow — cardinal, rummy-nose, lemon or black skirt. Note that the black skirt is itself a fin nipper and angelfish trail long fins, so it is a trade rather than a clean win.
My tank isn't cycled. Can I add fish anyway?
New tank syndrome kills more aquarium fish than overstocking does. Until the filter carries enough bacteria to convert ammonia to nitrite and nitrite to nitrate, every fish in there is being burned by its own waste — and the damage to gills and organs is often permanent in survivors. A fishless cycle with bottled ammonia takes three to six weeks. Here is how to run one.
Does a planted tank let me keep more fish?
Somewhat. Growing plants consume ammonia and nitrate directly, and this tool allows about 15% more on the volume gauge for it. Plants add no surface area for gas exchange, and at night they consume oxygen rather than producing it, so they do not move the oxygen gauge at all.
Why does this disagree with my fish shop or a forum thread?
Because compatibility is genuinely contested, and because a shop is selling fish. Where the hobby disagrees, the species note says so rather than pretending to certainty, and where we are unsure we flag a caution instead of a tick. Adult size and minimum tank length are the two figures most often understated at the point of sale — a common pleco is sold at three inches and reaches eighteen.
Can I count my sump or my hang-on-back as extra volume?
Sump water is real water and does dilute nitrate, so add it if you want the volume gauge to reflect reality. It does not add gas exchange unless the sump is open to air and turbulent, and it never adds footprint.
Does pH matter as much as people say?
Stability matters more than the number. Most commercially bred fish adapt to a wide range if it is steady, and chasing a target with pH-adjusting chemicals kills more fish than the original reading would have. The genuine conflicts are at the extremes — soft blackwater species alongside hard-water livebearers, for instance.