Aquarium Stocking Calculator
Add your tank size and the fish you want — AquaGauge shows live stocking % and filtration %, and flags overstocking, aggression, schooling and minimum-tank-size problems. It uses a per-species bioload model, not the inaccurate “1 inch per gallon” rule.
Stocking — under 85% comfortable · 85–100% full · over 100% overstocked
Show the math
Is my tank overstocked?
Your tank is overstocked when the combined bioload of your fish exceeds the volume that supports it, which is anything above 100% on the gauge. The three bands this calculator uses are:
- Under 85%, comfortable. Headroom for growth, for a fish you did not plan on, and for the week you skip a water change.
- 85 to 100%, full. Workable, but it depends on strong filtration and water changes that actually happen on schedule.
- Over 100%, overstocked. Ammonia and nitrate climb faster than the filter and your water changes clear them. Remove fish or move up a tank size.
Filtration is scored separately against a 4× turnover target, so a tank can read comfortable on stocking and still be flagged underpowered. Both have to be right.
Percentages are a planning guide, not a verdict on a tank you already own. A 95% tank with a mature filter, live plants and weekly changes is healthier than a 60% tank that was never cycled. If a reading surprises you, test for ammonia before you rehome anything.
Why do stocking calculators disagree?
Run the same fish through two calculators and you will often get two different answers, because they are not measuring the same thing. Most of the difference comes down to three choices:
- The model. Some still use surface area or the “1 inch of fish per gallon” rule. This one uses a per-species bioload value, so a 3-inch goldfish and three 1-inch tetras are not treated as equivalent. The goldfish produces far more waste.
- What counts as full. Some tools call 100% the maximum a filter can process; others call it a comfortable ceiling. Same tank, different label.
- Filtration. Some fold filter capacity into one score. This calculator keeps stocking and filtration as two separate gauges, because they fail in different ways and are fixed in different ways.
None of them is measuring your actual nitrogen cycle. Treat any calculator, including this one, as a plan to sanity-check with a test kit.
What size tank do real keepers actually run?
From the AquaGauge dataset: 5,134 real tanks that stated a volume, compiled from aquarium subreddits (July 2026 snapshot). How we collect this.
The median real tank is 20 gallons (76 L), and 58% are 20 gallons or smaller. The mean is higher at 39 gallons, pulled up by a long tail of large tanks; the largest in the dataset is 1200 gallons. Most stocking questions are therefore about small tanks, where a single extra fish moves the percentage a lot.
| Tank size | Share of real tanks |
|---|---|
| Under 5 gallons | 4% |
| 5 gallons (19 L) | 14% |
| 6 to 9 gallons | 3% |
| 10 gallons (38 L) | 18% |
| Over 10 gallons | 61% |
Middle 50% of tanks: 10 to 50 gallons (38–189 L). Tanks are self-reported by keepers who chose to post, so read this as what the hobby talks about, not a census of what it owns.
How this calculator works
Every species carries a bioload — a unit of biological waste load per adult fish. We sum the bioload of all your fish and divide by your tank volume (in gallons) to get the stocking %. Filtration % compares your filter’s GPH to a ~4× turnover target. It then checks each fish against its minimum tank size, schooling minimum, temperament and temperature range. This replaces the flawed inch-per-gallon rule. Full model on our methodology page.
Species data used
Curated starter set (31 common freshwater species). Bioload and ranges are tuned against community norms; we are expanding the database.
| Species | Adult size | Min group | Min tank | Temperament | Temperature |
|---|---|---|---|---|---|
| Neon tetra Paracheirodon innesi | 1.5″ (3.8 cm) | 6+ | 10 gal (38 L) | peaceful | 70–81 °F (21–27 °C) |
| Ember tetra Hyphessobrycon amandae | 0.8″ (2 cm) | 6+ | 10 gal (38 L) | peaceful | 73–84 °F (23–29 °C) |
| Cardinal tetra Paracheirodon axelrodi | 1.2″ (3 cm) | 6+ | 15 gal (57 L) | peaceful | 73–81 °F (23–27 °C) |
| Harlequin rasbora Trigonostigma heteromorpha | 1.8″ (4.6 cm) | 6+ | 10 gal (38 L) | peaceful | 72–81 °F (22–27 °C) |
| Guppy Poecilia reticulata | 2″ (5.1 cm) | 3+ | 10 gal (38 L) | peaceful | 72–82 °F (22–28 °C) |
| Endler's livebearer Poecilia wingei | 1.4″ (3.6 cm) | 3+ | 10 gal (38 L) | peaceful | 72–82 °F (22–28 °C) |
| Platy Xiphophorus maculatus | 2.5″ (6.4 cm) | 3+ | 10 gal (38 L) | peaceful | 70–82 °F (21–28 °C) |
| Molly Poecilia sphenops | 4″ (10.2 cm) | 3+ | 20 gal (76 L) | peaceful | 72–82 °F (22–28 °C) |
| Zebra danio Danio rerio | 2″ (5.1 cm) | 6+ | 10 gal (38 L) | semi-aggressive | 64–77 °F (18–25 °C) |
| White cloud minnow Tanichthys albonubes | 1.5″ (3.8 cm) | 6+ | 10 gal (38 L) | peaceful | 60–72 °F (16–22 °C) |
| Corydoras catfish Corydoras sp. | 2.5″ (6.4 cm) | 6+ | 20 gal (76 L) | peaceful | 72–80 °F (22–27 °C) |
| Pygmy corydoras Corydoras pygmaeus | 1″ (2.5 cm) | 6+ | 10 gal (38 L) | peaceful | 72–79 °F (22–26 °C) |
| Panda corydoras Corydoras panda | 2″ (5.1 cm) | 6+ | 20 gal (76 L) | peaceful | 68–77 °F (20–25 °C) |
| Bronze corydoras Corydoras aeneus | 3″ (7.6 cm) | 6+ | 20 gal (76 L) | peaceful | 72–80 °F (22–27 °C) |
| Sterbai corydoras Corydoras sterbai | 2.6″ (6.6 cm) | 6+ | 20 gal (76 L) | peaceful | 75–82 °F (24–28 °C) |
| Peppered corydoras Corydoras paleatus | 2.8″ (7.1 cm) | 6+ | 20 gal (76 L) | peaceful | 64–78 °F (18–26 °C) |
| Julii corydoras Corydoras julii | 2.2″ (5.6 cm) | 6+ | 20 gal (76 L) | peaceful | 72–79 °F (22–26 °C) |
| Otocinclus Otocinclus sp. | 1.5″ (3.8 cm) | 6+ | 20 gal (76 L) | peaceful | 72–79 °F (22–26 °C) |
| Kuhli loach Pangio kuhlii | 3.5″ (8.9 cm) | 5+ | 20 gal (76 L) | peaceful | 73–86 °F (23–30 °C) |
| Dwarf gourami Trichogaster lalius | 3.5″ (8.9 cm) | — | 10 gal (38 L) | semi-aggressive | 72–82 °F (22–28 °C) |
| Tiger barb Puntigrus tetrazona | 2.8″ (7.1 cm) | 6+ | 20 gal (76 L) | semi-aggressive | 74–82 °F (23–28 °C) |
| Serpae tetra Hyphessobrycon eques | 1.6″ (4.1 cm) | 6+ | 15 gal (57 L) | semi-aggressive | 72–79 °F (22–26 °C) |
| Betta (male) Betta splendens | 2.5″ (6.4 cm) | — | 5 gal (19 L) | aggressive | 76–82 °F (24–28 °C) |
| Angelfish Pterophyllum scalare | 6″ (15.2 cm) | — | 29 gal (110 L) | semi-aggressive | 76–84 °F (24–29 °C) |
| Discus Symphysodon aequifasciatus | 5.5″ (14 cm) | 5+ | 55 gal (208 L) | semi-aggressive | 82–86 °F (28–30 °C) |
| Goldfish (fancy) Carassius auratus | 8″ (20.3 cm) | — | 30 gal (114 L) | peaceful | 65–72 °F (18–22 °C) |
| Cherry shrimp Neocaridina davidi | 0.6″ (1.5 cm) | 6+ | 5 gal (19 L) | peaceful | 65–80 °F (18–27 °C) |
| Nerite snail Neritina sp. | 1″ (2.5 cm) | — | 5 gal (19 L) | peaceful | 65–85 °F (18–29 °C) |
| Mystery snail Pomacea bridgesii | 2″ (5.1 cm) | — | 5 gal (19 L) | peaceful | 68–82 °F (20–28 °C) |
| Bristlenose pleco Ancistrus sp. | 5″ (12.7 cm) | — | 20 gal (76 L) | peaceful | 70–79 °F (21–26 °C) |
| Common pleco Hypostomus plecostomus | 15″ (38.1 cm) | — | 75 gal (284 L) | peaceful | 72–82 °F (22–28 °C) |
FAQ
- How does the stocking calculator work?
- Each species has a bioload value (waste load per fish). We add the bioload of every fish you select and compare it to your tank size; ~100% means fully stocked. We also check filter turnover, schooling minimums, minimum tank size and temperament.
- Is the “1 inch of fish per gallon” rule accurate?
- No. It ignores adult size, body mass, waste output and swimming space. AquaGauge uses a per-species bioload model instead.
- What stocking percentage is safe?
- Aim to stay at or below ~85–100%. Under 85% gives comfortable headroom; over 100% risks ammonia spikes unless filtration and water changes are excellent.
- How much filter flow (GPH) do I need?
- A common target is about 4× your tank volume per hour — e.g. ~40 GPH for a 10-gallon tank. Heavily stocked tanks benefit from more.
- How do I know if my tank is overstocked?
- Add your fish above: over 100% on the stocking gauge means the combined bioload exceeds what the volume supports. Under 85% is comfortable, 85 to 100% is full but workable with strong filtration and regular water changes. Filtration is scored separately, so a tank can look comfortable on stocking and still be flagged underpowered. Confirm with an ammonia test rather than trusting a percentage alone.
- Why do stocking calculators give different answers?
- Because they use different models. Some still apply the "1 inch of fish per gallon" rule or a surface-area formula, while this one uses a per-species bioload value, so a goldfish and three neon tetras of the same total length are not treated as equal. Tools also disagree on what 100% means — a filter ceiling for some, a comfortable ceiling for others — and on whether filtration is folded into one score or kept separate, as it is here.
- Can I use litres instead of gallons?
- Yes. Switch the Units selector to Litres and the tank size, filter flow, fish sizes and every warning switch with it. The underlying bioload model is defined per US gallon, so the calculator converts your litres before it computes and shows that conversion under "Show the math".