Quick answer: This aquarium stocking calculator shows how full your tank would be with the fish you pick, based on their adult size and how much waste they make, not the size they are in the store. Add your fish, and it gives you a stocking percentage plus warnings about tanks that are too small, groups that are too few, fin nippers with long-finned fish, aggressive fish, predators, goldfish with tropicals and temperatures that don’t overlap. The common mistake is the old “one inch of fish per gallon” rule. It treats a 12 inch oscar like twelve neon tetras, which badly underestimates big, messy fish.
Not sure of your real volume? Use our aquarium volume calculator first.
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Estimates for a cycled tank. Adult sizes are used, not store sizes. Bioload is weighted, so heavy waste producers like goldfish and plecos count for more than their length. Always watch your own fish and test your water.
How to use the fish stocking calculator
Enter your real water volume. Use the water actually in the tank, not the size on the box. A 20 gallon with sand, rocks and wood often holds 14 to 17 gallons. Our aquarium volume calculator works it out in a few seconds.
Pick your filtration and water changes. A strong filter, lots of live plants and big weekly water changes let a tank handle a little more. A small filter with few plants handles less.
Add each species and how many. The calculator fills in the minimum group size for schooling fish automatically. Change the number if you need to, then click Add to tank.
Read the result and the warnings. The color bar shows your stocking level. Green, up to 80 percent, is comfortable. Orange is full. Red means too much waste for the tank. The warnings below explain any compatibility problems, and fish with a care guide on Aqua Planet link straight to it.
Stocking levels at a glance
| Result | What it means | What to do |
|---|---|---|
| Under 50% | Lightly stocked | Lots of room. Add fish slowly, a few at a time |
| 50 to 80% | Good stocking level | The sweet spot for a healthy, easy tank |
| 80 to 100% | Fully stocked | Stop adding fish and keep up weekly water changes |
| Over 100% | Overstocked | Remove fish, upgrade filtration or move to a bigger tank |
Why the inch per gallon rule doesn’t work
The “one inch of fish per gallon” rule is easy to remember, which is why it’s everywhere. Here’s the honest part most stocking guides skip: it only works roughly for small, slim fish. A fish that’s twice as long is far more than twice as heavy, so it eats more and makes far more waste. One 6 inch fish produces much more waste than six 1 inch fish.
| Fish | Adult size | What the inch rule says | The reality |
|---|---|---|---|
| 10 neon tetras | 1.5 in each | 15 gallons | Fine in a 20 gallon with good filtration |
| 1 bristlenose pleco | 5 in | 5 gallons | Needs at least 20 gallons and makes heavy waste |
| 2 fancy goldfish | 7 in each | 14 gallons | Need about 30 gallons and strong filtration |
| 1 oscar | 12 in | 12 gallons | Needs 75 gallons or more |
That’s why this calculator weights each fish by size and waste. Goldfish, plecos and big cichlids count for more than their length, and small invertebrates like shrimp and snails count for very little.
What the calculator checks
Minimum tank size. Every fish has a minimum, based on its adult size and how much it swims. Tiger barbs need about 30 gallons, for example, even though they’re small, because they’re fast and active.
Group size. Schooling fish like tetras, rasboras and corydoras get stressed and hide in small numbers. The calculator warns you if a group is below the usual minimum, normally 6.
Fin nippers and long fins. Tiger barbs, serpae tetras and black skirt tetras nip at long, flowing fins. Bettas, guppies and angelfish are the usual victims.
Aggressive fish. Convicts, oscars and green terrors will bully or kill small peaceful fish. Read our green terror cichlid guide before mixing big cichlids with anything else.
Predators. Angelfish, oscars and rope fish eat anything small enough to fit in their mouth, and that includes neon tetras once the angelfish is grown.
Temperature. It shows the temperature range that suits every fish you added. If there’s no overlap, at least one species will be stressed. Goldfish and white cloud minnows like cool water, while discus and rams want it very warm.
Shrimp and snails. It warns when fish in your list are likely to eat cherry shrimp or snails. Pea puffers and clown loaches are famous snail hunters.
Example stocking ideas
| Tank | Example stocking | Result |
|---|---|---|
| 10 gallon | 8 chili rasboras, 6 pygmy corydoras, 3 Amano shrimp | About 70%, a peaceful nano tank |
| 20 gallon long | 10 neon tetras, 6 panda corys, 1 honey gourami, 3 Amano shrimp | About 75%, a good community |
| 29 gallon | 8 harlequin rasboras, 6 bronze corys, 1 pearl gourami, 1 bristlenose pleco | About 90%, fully stocked |
| 55 gallon | 12 rummynose tetras, 8 corydoras, 1 pair of angelfish, 1 bristlenose pleco | About 80%, a classic planted community |
For more beginner-friendly species, see our best aquarium fish for beginners list. For the gentle bottom of a community, our pygmy corydoras and bristlenose pleco guides are a good place to start.
How to add fish safely
- Cycle the tank first. Never add a full stocking list to a new tank. Our how to cycle a fish tank guide explains how.
- Add fish in batches. Start with the hardiest fish, then add more every 1 to 2 weeks so the bacteria can catch up.
- Add peaceful fish before territorial ones. Territorial fish added first tend to claim the whole tank.
- Test the water. Ammonia and nitrite should stay at 0 after each new batch.
- Quarantine if you can. A separate tank for 2 to 4 weeks catches disease before it spreads.
Common stocking mistakes
Buying fish by store size. A 2 inch common pleco will grow to well over a foot. Always check adult size. Our pleco fish guide shows how different pleco types compare.
Too few schooling fish. Two or three neon tetras will hide and stress. Six is the usual minimum, more is better.
Mixing goldfish with tropicals. Goldfish want cool water and make huge amounts of waste.
Adding everything at once. Even a well-planned list can crash a tank if it all goes in on day one.
Forgetting the cleanup crew counts. Plecos especially add a lot of waste. Shrimp and snails add very little.
Once your stocking is set, our aquarium fertilizer calculator helps you feed the plants that keep that waste in check.
FAQ
How many fish can I put in a 20 gallon tank?
It depends on the fish. A 20 gallon can comfortably hold about 10 small tetras, 6 small corydoras and one centerpiece fish like a honey gourami. Larger or messy fish like goldfish or plecos take up much more of that space.
Is the one inch per gallon rule accurate?
Only roughly, and only for small, slim fish. It badly underestimates large fish, because waste grows much faster than length.
How do I know if my tank is overstocked?
Signs include ammonia or nitrite showing up, nitrate climbing fast between water changes, cloudy water, algae, and fish fighting or gasping. This calculator flags it before you buy.
How many neon tetras can go in a 10 gallon tank?
Around 6 to 8 neon tetras in a 10 gallon is a good number, especially with live plants. Leave some room if you also want shrimp or a few small corydoras.
Do shrimp and snails count toward stocking?
A little. They produce very little waste compared with fish, so the calculator counts them lightly, but mystery snails and large groups still add up.
Can I add all my fish at once?
No. Add them in small batches every 1 to 2 weeks in a cycled tank, so the beneficial bacteria can grow with the extra waste.