Aquarium Stocking Calculator
Last updated: October 6, 2026
This calculator turns a tank size and adult fish counts into stocking points, and it also plans heater watts, filter gallons per hour, and a water-change percent. On a normal setup, capacity is 1 point per gallon (small fish 1, medium 3, large 8, XL 14). A 20-gallon tank from 68°F to 78°F is 100 watts.
Free tank recipe card — print it, 10 minutes.
Stocking calculator for freshwater aquarium keepers.
Bioload beats the inch-per-gallon myth. Count your fish by adult size and waste class — get stocking points, heater watts, filter GPH, and your water-change schedule.
As an Amazon Associate I earn from qualifying purchases. The Amazon links below are affiliate links; they don't change the math.
Test kits and filters on Amazon. liquid test kits on Amazon · HOB filters on Amazon · heaters on Amazon
Why does bioload beat the inch-per-gallon rule?
The 1-inch-per-gallon rule dates from goldfish-bowl era advice. It ignores body mass, and it ignores that surface area limits oxygen exchange more than volume limits waste. Adult size is what counts. Bioload points on this calculator are small (≤2") = 1 pt, medium (2–5") = 3 pts, large (5–10") = 8 pts, XL (10"+) = 14 pts. A normal setup allows 1 point per gallon. A heavy-stock setup allows 0.6 points per gallon.
Methodology & disclosure: the calculator applies the published rules of thumb on this page in your browser. It is a planning estimate, not a measured result — test your water, don't guess. Amazon links on this site are affiliate links; they do not change the math.
What stocking capacity, heater, and filter does each tank size need?
| Tank | Capacity (pts) | Heater, 68°F to 78°F | Filter rating, normal |
|---|---|---|---|
| 10 gal | 10 | 50 W | 60 |
| 20 gal | 20 | 100 W | 120 |
| 29 gal | 29 | 150 W | 175 |
| 55 gal | 55 | 250 W | 330 |
| 75 gal | 75 | 300 W | 450 |
Capacity is this calculator on a normal setup. Heater watts are the same function for a 10°F rise, rounded up to a standard size. Filter ratings are the normal level: 5 turnovers, then a 20% step rounded up to the next 5 gallons per hour. Tanks of 40 gallons and up also get a two-heater note. These are planning estimates, not a published standard.
Sourced data table + CSV: freshwater fish adult size and minimum tank footprint
Common questions, answered
Is 1 inch of fish per gallon a good stocking rule?
No — it fails at both ends. A 10-inch oscar in a 10-gallon tank 'fits' the rule and dies in weeks; a dozen 1-inch neon tetras in 10 gallons is lightly stocked. Bioload scales with body mass and waste output, not length. Count adult size, use bioload points (small fish 1, medium 3, large 8, XL 14), and keep the total at or under capacity: 10 points per 10 gallons on a normal setup.
What wattage heater do I need for my aquarium?
For a rise above 9°F and at most 15°F, this calculator uses 4 watts per gallon and rounds up to the next standard size. A 20-gallon tank from 68°F to 78°F is 100 watts, one heater. A 55-gallon tank from 68°F to 78°F is 250 watts, and the two-heater note is 125 watts each.
How strong should my aquarium filter be?
On this calculator, 29 gallons at the normal level is 5 turnovers an hour, 145 gallons per hour ideal, and a rating of 175. The rating adds a 20% step and rounds up to the next 5 gallons per hour. That step is this site's allowance, not a manufacturer measurement.
How often should I change aquarium water?
On this calculator, a weekly nitrate rise of 20 ppm, a ceiling of 40 ppm, and tap water at 5 ppm returns a 60% water change. The plateau at that change is 38 ppm. Those three figures are example inputs, not a test from a tank.
Guides
How many fish per gallon? The honest answer
Why the inch rule fails, how bioload points work, and worked examples for 10, 20, and 55 gallon tanks.
Aquarium heater wattage
20 gallons from 68°F to 78°F is 100 watts on this calculator. Aqueon, EHEIM, and Fluval do not all agree.
Heater and filter size chart
Maker watts and rated flow, copied as published. Differing charts stay on separate rows.
Power outages and your tank ↗
A stocked tank on a battery backup — GeneratorSizer and Battery Bank Sizer do the runtime math.
How to fix cloudy water in a new tank
Test ammonia and nitrite before treating a cloud. The Merck timelines stay separate.
How to add fish
At least 30 days of quarantine, then a first group the calculator can hold.
How much to feed tropical fish
One feeding a day for most pets. Koi and goldfish are not on that schedule.
How to treat sick fish
Test the water first. No drug doses on the page.
How to keep pet fish alive
One 20-gallon starter run through stocking, heat, filter, and water changes.
How many fish in a 10, 29, or 55 gallon tank
Each size through the stocking, heater, and filter functions. Comfortable is not a day-one load.
How often to change aquarium water
No universal weekly percent. Example inputs of 20, 40, and 5 ppm return 60% on this calculator.
How many koi in a 500-gallon pond
The aquarium formula applied to 500 gallons, labeled as such. SRAC has no home-pond headcount.
How to humanely euthanize a fish
AVMA 2020 wording. A veterinarian does it. Flushing and an unanesthetized freezer are unacceptable.
Common questions
Each question below is its own page. The calculator on this page is the place to check a mixed stocking list.
Heater size
- What size heater does a 10 gallon tank need?
- What size heater does a 29 gallon tank need?
- What size heater does a 40 gallon tank need?
- What size heater does a 55 gallon tank need?
- What size heater does a 75 gallon tank need?
Small schooling fish
- What size tank do neon tetras need?
- What size tank do cherry barbs need?
- What size tank do harlequin rasboras need?
- What size tank do zebra danios need?
Livebearers and labyrinth fish
- What size tank does a betta fish need?
- What size tank do guppies need?
- What size tank do platies need?
- What size tank do mollies need?
- What size tank does a dwarf gourami need?
Bottom dwellers
Cichlids
Sources & Standards
Bioload points, watts per gallon, and filter turnover on these pages are planning estimates for a cycled freshwater tank. They are not a published standard.
Stocking a 4 ft tank? See how many fish in a 75 gallon tank.