Fish Stocking Calculator

How Many Fish Can My Tank Hold?

Overstocking is one of the most common mistakes beginner aquarists make. Too many fish leads to spikes in ammonia and nitrite, increased disease, stressed fish, and a tank that's much harder to maintain. This calculator helps you avoid that by checking two widely-used stocking methods at once.

The Legacy Inch-Per-Gallon Rule (and Its Limits)

The simplest way to estimate stocking: allow 1 inch of adult fish body length per US gallon of water. A 20-gallon tank could hold 20 inches worth of fish — that might be 10 neon tetras (Paracheirodon innesi) at 1.5 inches each, or 4 platies (Xiphophorus maculatus) at 2.5 inches each. We show it only as a rough cross-check: it treats a slim tetra and a deep-bodied goldfish (Carassius auratus) as equivalent, and ignores filtration performance and swimming terrain entirely — the factors that matter most in a healthy stocking plan.

Swimming Terrain: The Surface Area Method

A more accurate approach: allow 1 inch of fish per 12 square inches of surface area. This is because oxygen exchange happens at the water surface — more surface area means more oxygen, which means more fish can be supported. A 24 × 12 inch tank has 288 in² of surface area, allowing up to 24 inches of fish. This method rewards tanks with larger footprints over taller tanks.

When you enter your tank’s dimensions, the calculator shows both results and bases its overall verdict on the more conservative of the two — better to under-promise on a screening tool than to green-light an overstocked tank. Both rules also count every inch of fish equally, which over-penalises nano fish and invertebrates; the bioload calculator weighs body mass instead and is the fairer judge for those.

Swimming Levels Help With Space — Not Compatibility

Distributing your fish across the top, middle, and bottom of the tank is a genuinely useful planning trick: it spreads swimming room and territory through the whole water column instead of crowding every fish into one layer. A school of mid-water tetras, a surface-dwelling gourami, and a group of bottom-dwelling corydoras each use a different slice of the same footprint, so the tank you already own goes further. That is why the calculator tracks which level each fish occupies and shows the spread in the Tank Distribution summary. It is a space-planning aid, nothing more.

What it does not do is make incompatible fish compatible. A territorial species defends its patch whether rivals swim above it or below it; a predator big enough to swallow a neon tetra will usually hunt at every level; a fin-nipper follows the flowing fins, not the depth. Peaceful coexistence depends on aggression, territorial behaviour, predation risk, adult size, and whether a species needs a school to feel secure — none of which change with swimming depth. Before mixing any two species, read both care sheets in the fish database and check temperament, adult size, and schooling needs directly. Different swimming levels make better use of the space you have — they do not automatically make a community safe.

Bioload Capacity & Filtration Performance

A heavily planted tank with lots of live plants naturally processes more waste and produces more oxygen, allowing you to stock slightly more fish. A predator or aggressive tank, on the other hand, requires more space per fish to reduce territorial conflict and stress — so we apply a 30% reduction to the maximum capacity.

Why There Is No Single Perfect Stocking Number

If you take one idea away from this page, make it this: tank volume alone does not decide how many fish you can keep. Two aquariums with identical gallon counts can suit completely different stocking levels. A 20-gallon tank holding slim, peaceful, schooling tetras is a very different environment from the same 20 gallons holding a single deep-bodied, territorial cichlid — even though the number on the label is the same. Any tool that hands you one perfect number is overselling what volume alone can tell you.

In practice, the right stocking level depends on a cluster of factors working together: the adult size of every species, because a 5 cm juvenile can grow into a 30 cm adult; body shape, because a deep-bodied fish displaces far more water than a slim one of the same length; swimming behaviour, because active swimmers need tank length while sedate species do not; territoriality, which claims floor space that no volume formula sees; schooling needs, because six tetras is a group and two is a stress problem; filtration capacity and your maintenance routine, which set how much waste the system actually processes; and the physical footprint of the tank, which controls oxygen exchange and swimming room independently of its height.

So treat every number this calculator gives you — including the ones in the worked examples below — as a suggested starting point rather than a verdict. Cross-check both methods, read the species profile of every fish you plan to add, under-stock when you are unsure, and use the bioload calculator for a mass-based second opinion on nano fish and invertebrates. A number gets you started; observation, water testing, and gradual stocking are what keep the tank healthy over the long run.

Worked Stocking Examples — Real Tank Plans

The best way to understand how the calculator thinks is to walk through real stocking plans. Here are four common aquarium configurations with the fish you would enter and the results you should expect. These plans come from tanks I’ve actually run and tested — they’ve held up well for me, because the bioload, surface area, and terrain all stay in balance.

Example 1: 10-Gallon Nano Community

Enter a 10-gallon tank (10 gal, 20×10×12 in), set the tank type to “Community”, and add: 6 Ember Tetras (Mid · Nano · Slim) and 1 Betta (Top · Medium · Average). The calculator reports roughly 70% on the inch-per-gallon method and about 40% on surface area — a comfortable, sustainable load for a 10-gallon with a sponge filter and live plants. The ember tetras (Hyphessobrycon amandae) school in the mid-water while the betta holds the surface. Want pygmy corydoras (Corydoras pygmaeus) or cherry shrimp (Neocaridina davidi) in there too? Length-based rules over-count small fish and invertebrates — which is where the bioload calculator gives the fairer, mass-based verdict. See more 10-gallon stocking combinations here.

Example 2: 20-Gallon Long Schooling Tank

Enter a 20-gallon long (16.8 gal real volume, 30×12×12 in), set the tank type to “Community”, and add: 8 Neon Tetras (Mid · Small · Slim) and 6 Pygmy Corydoras (Bottom · Nano · Average). The calculator reports roughly 90% on the inch-per-gallon method and about 50% on surface area — fully stocked, with no room for more fish without upgraded filtration. The 30-inch length of a 20-long gives the schooling fish room to display proper group behaviour. A hang-on-back filter rated for 30–40 gallons handles the bioload comfortably; if you want the fuller version of this tank (harlequins plus a bristlenose (Ancistrus sp.)), the bioload calculator is the tool that weighs it fairly.

Example 3: 55-Gallon Peaceful Community

Enter a 55-gallon tank (45.7 gal real volume, 48×13×21 in), set the tank type to “Community” and the filter to “Canister”, and add: 12 Rummy Nose Tetras (Mid · Small · Slim), 6 Corydoras (Bronze) (Bottom · Small · Average), 1 Bristlenose Pleco (Bottom · Large · Average), and 4 Pearl Gouramis (Mid · Medium · Deep). The calculator reports roughly 85% on the inch-per-gallon method and about 75% on surface area — a fully stocked, balanced community. A canister filter rated for 75+ gallons is appropriate for this load. Amano shrimp (Caridina multidentata) barely register on a length-based screen — add them via the bioload calculator instead.

Example 4: 5-Gallon Betta & Shrimp

Enter a 5-gallon tank (3.7 gal real volume, 16×8×10 in), set the tank type to “Community”, and add: 1 Betta (Top · Medium · Average). The calculator reports roughly 80% on the inch-per-gallon method — appropriate for a small tank where stability matters more than density. The betta gets the tank to itself (no fin-nipping tankmates in a 5-gallon). Cherry shrimp make the classic cleanup crew here: they barely register on a length-based screen and score near zero on the bioload calculator, which is the better judge for invertebrates. A sponge filter is a good choice for filtration — gentle current, no intake to suck up shrimp.

Every species named in these examples has a full care sheet — adult size, parameters, diet, tank mates and breeding — in our fish database. Reading the profile before you buy is what separates a stocking plan that works from one that quietly fails eight months later when the juvenile fish reach adult size.

Sample Stocking Plans by Tank Size

Below are field-tested stocking plans for the most common aquarium sizes. Each plan assumes a cycled tank, a filter rated for the stated volume, and at least 30% live plant cover. Use these as starting points — adjust based on your own water parameters, filtration, and the specific species you want to keep.

Each plan above has been tested over multiple generations in a working fishroom. They hold up because they respect three principles: enough school size for each species to feel secure, layered use of the water column (surface, mid-water, substrate), and a bioload the stated filtration can comfortably process. Layering spreads out space use, not temperament — whether the mix actually gets along still depends on the species involved, so check temperament before assuming two fish can share a tank just because they swim at different levels. Note that the conservative inch-per-gallon screen reads several of these plans as tight on smaller tanks — nano fish and invertebrates are the very cases length-based rules over-count — so run anything borderline through the bioload calculator for the mass-based verdict before you commit. When in doubt, under-stock — a 70%-full tank is nearly always easier to maintain than a 95%-full one.

How to Add Fish Gradually (and Why You Must)

Even a carefully planned stocking plan can fail if you add all the fish at once. A cycled filter contains enough beneficial bacteria to process the ammonia from a certain bioload — add more fish than that, and ammonia spikes before the bacteria can multiply to catch up. The standard approach is to add fish in waves, with a week or two between each wave, and to test ammonia and nitrite daily during the ramp-up:

1. Week 1: Add the most sensitive species first (usually the schooling fish). Test ammonia and nitrite daily. Both should read 0. If either spikes, do a 50% water change and wait a week before adding more.
2. Week 2–3: Add the bottom-dwellers (corydoras, loaches, plecos). The filter bacteria have grown to handle the first wave; this second wave should also process cleanly.
3. Week 3–4: Add cleanup crew (shrimp, snails). These have a low bioload but benefit from a mature tank with stable parameters and biofilm.
4. Always last: Add centrepiece or territorial fish (bettas (Betta splendens), gouramis, cichlids). They establish territories more peacefully in a tank that already has other fish settled in.

If at any point during the ramp-up you see ammonia or nitrite above 0.25 ppm, stop adding fish and do water changes until the parameters stabilise. The cycling guide explains the underlying biology in detail; the short version is that bacteria multiply slowly, and the only dependable way to increase bioload is gradually.

What Stocking Method Is Best for Your Tank?

Different tank types call for different stocking approaches. The calculator adjusts its recommendations based on the tank type you select, but here’s the reasoning behind each one:

• Community tank (most common): Standard stocking capacity. Multiple species, peaceful temperaments, mixed water-column use. Aim for 70–90% capacity — enough fish for a lively display, but with a safety margin for water-change lapses.
• Species-only tank: Reduced stocking capacity. A single-species tank (just bettas, just shrimp, just endlers (Poecilia wingei)) doesn’t benefit from the layered bioload distribution of a community. Aim for 60–75% capacity to keep the single species healthy and breeding.
• Predator/aggressive tank: Significantly reduced capacity (30% reduction). Cichlids, puffers, and predatory catfish need territory and swimming space far beyond what their bioload alone would suggest. The calculator applies the reduction automatically when you select “Predator” as the tank type.
• Heavily planted tank: Slightly increased capacity. Live plants process a meaningful amount of nitrogen waste directly, so the calculator applies a 20% capacity increase when you select “Planted” as the tank type — the plants are part of the filtration system.

The calculator bakes these adjustments in automatically. If you’re unsure which tank type to choose, start with “Community” — it’s the sensible default and the one most beginners should be using.

Frequently Asked Questions