How Many Fish Can I Put in My Tank?
Getting your stocking level right is the single most important factor in keeping a healthy aquarium. Too many fish and you risk ammonia spikes, oxygen depletion, and chronic stress. Too few and your fish may become anxious, hide constantly, or show aggression. Our aquarium stocking calculator uses a modern bioload-based method — far more accurate than the old 1-inch-per-gallon rule — to give you a reliable stocking limit for any tank.
How to Use the Stocking Calculator
The calculator runs entirely in your browser — no sign-up, no loading screens. Here is how to get the most accurate result:
Enter your tank volume
Type your tank size in US gallons or litres. If you are unsure of the exact volume, use our tank volume calculator first to measure by dimensions.
Select water type and tank shape
Freshwater, saltwater, and brackish each have distinct fish communities. Tall or cylindrical tanks have less oxygenated surface area than long shallow tanks of the same volume — the shape multiplier accounts for this.
Set your filtration quality
An oversized canister filter or a heavily planted setup can support more bioload than a basic HOB. Select the option that honestly reflects your setup — over-estimating this is the most common stocking mistake.
Browse and add fish
Filter by category or search by name. Each fish card shows its adult size, bioload points, and school minimum. Click + to add to your tank and adjust quantities in the Your Tank panel.
Read the compatibility panel
The calculator automatically checks for aggression conflicts, fin-nipping risk, temperature incompatibilities, schooling minimums, and overstocking. Address any red warnings before purchasing fish.
Aquarium Stocking Calculator
Configure your tank below, then browse the fish library to build your community. The stocking meter and compatibility panel update in real time as you add or remove fish.
Add fish to see compatibility notes.
What Is Bioload and Why Does It Matter?
Bioload is the total amount of organic waste — primarily ammonia — that the inhabitants of your tank produce. When fish respire and excrete waste, ammonia is released into the water. Beneficial bacteria in your filter (the nitrogen cycle) convert this ammonia to nitrite and then to the less toxic nitrate. If waste production exceeds your filter’s processing capacity, ammonia and nitrite accumulate to harmful levels.
The 1-inch-per-gallon rule was designed for dealerships in the 1950s. It treats a 6-inch goldfish the same as six 1-inch neon tetras — yet a single goldfish produces more waste than a school of twenty neons.
Factors that increase bioload
Large body mass, messy eating habits (goldfish, cichlids, oscars), high metabolism, a protein-heavy diet, overstocked conditions, and infrequent water changes all raise the biological burden on your filter.
Factors that reduce effective bioload
Live plants consume ammonia and nitrate directly. Large filter media volume, oversized pumps, regular water changes, and low feeding frequency all help keep bioload under control.
Stocking Guide by Tank Size
The table below shows maximum bioload points and example community combinations for common aquarium sizes with standard filtration and rectangular shape. Upgrade your filter to unlock more capacity.
| Tank size | Max bioload pts | Stocking level | Example community |
|---|---|---|---|
| 5 gallon | 7 | Nano only | 8× Chili Rasbora or 1× Betta |
| 10 gallon | 15 | Small community | 8× Neon Tetra + 4× Corydora |
| 20 gallon | 30 | Community | 8× Cardinal Tetra + 6× Corydora + 2× Dwarf Gourami |
| 29 gallon | 43 | Community | 10× Rummynose + 6× Corydora + 2× Gourami + 1× Bristlenose |
| 40 gallon | 60 | Medium community | 10× Rummynose + 6× Corydora + 1 pair Kribensis + 1× Bristlenose |
| 55 gallon | 82 | Full community | 10× Cardinal + 8× Harlequin + 6× Corydora + 2× Pearl Gourami + 1× Bristlenose |
| 75 gallon | 112 | Spacious community | 4× Angelfish + 10× Rummynose + 8× Corydora + 6× Clown Loach |
| 125 gallon | 187 | Large display | 6× Discus + 10× Cardinal + 8× Corydora + Planted biotope |
Always aim to stock at 60–75% of your maximum bioload to leave headroom for growth, fry, and filter resilience. The stocking meter turns amber at 80% — treat that as your hard ceiling unless you have exceptional filtration.
Bioload Points by Fish Species
The calculator uses bioload points to measure each species’ waste footprint rather than just body length. A single goldfish produces as much waste as 10 neon tetras despite being only 6× longer.
| Species | Adult size | Bioload pts | Min. school | Temperament |
|---|---|---|---|---|
| Neon Tetra | 1.5″ | 2 | 6 | Peaceful |
| Cardinal Tetra | 2″ | 2 | 6 | Peaceful |
| Ember Tetra | 0.8″ | 1 | 8 | Peaceful |
| Congo Tetra | 3.5″ | 4 | 6 | Peaceful |
| Black Skirt Tetra | 2.5″ | 3 | 6 | Fin nipper |
| Chili Rasbora | 0.7″ | 1 | 8 | Peaceful |
| Harlequin Rasbora | 2″ | 2 | 8 | Peaceful |
| Lambchop Rasbora | 1.5″ | 1 | 8 | Peaceful |
| Zebra Danio | 2″ | 2 | 6 | Peaceful |
| Celestial Pearl Danio | 1″ | 1 | 8 | Peaceful |
| Guppy | 2″ | 2 | 3 | Peaceful |
| Platy | 2.5″ | 3 | 3 | Peaceful |
| Molly | 3.5″ | 4 | 3 | Peaceful |
| Corydoras Catfish | 2.5″ | 3 | 6 | Peaceful |
| Otocinclus | 2″ | 2 | 4 | Peaceful |
| Bristlenose Pleco | 5″ | 8 | 1 | Peaceful |
| Common Pleco | 18″ | 25 | 1 | Large / messy |
| Pictus Catfish | 5″ | 7 | 2 | Predatory |
| Dwarf Gourami | 3.5″ | 4 | 2 | Peaceful |
| Honey Gourami | 2″ | 3 | 2 | Peaceful |
| Betta | 2.5″ | 5 | 1 | Aggressive |
| Angelfish | 6″ | 10 | 2 | Semi-aggressive |
| German Blue Ram | 3″ | 5 | 2 | Sensitive |
| Discus | 8″ | 15 | 4 | Expert |
| Oscar | 14″ | 22 | 1 | Aggressive |
| Tiger Barb | 3″ | 4 | 8 | Fin nipper |
| Cherry Barb | 2″ | 2 | 6 | Peaceful |
| Common Goldfish | 12″ | 20 | 2 | Messy |
| Clown Loach | 12″ | 12 | 4 | Peaceful |
| Kuhli Loach | 4″ | 3 | 3 | Peaceful |
| Cherry Shrimp | 1.2″ | 0.5 | 10 | Peaceful |
| Amano Shrimp | 2″ | 0.5 | 6 | Peaceful |
| Clownfish | 4″ | 8 | 2 | Peaceful |
| Pajama Cardinalfish | 3″ | 4 | 5 | Peaceful |
| Royal Gramma | 3″ | 4 | 1 | Peaceful |
| Blue Tang | 12″ | 18 | 1 | Active |
| Yellow Tang | 8″ | 12 | 1 | Reef safe |
| Lawnmower Blenny | 5″ | 5 | 1 | Peaceful |
| Coral Beauty Angelfish | 4″ | 7 | 1 | Semi-aggressive |
Fish Compatibility: What Can Live Together?
Stocking level is only half the puzzle. Even a correctly-stocked tank can be a disaster if the fish are incompatible. Aggression, predation, temperature mismatch, and pH differences all cause chronic stress and shortened lifespans. The compatibility panel above catches the most common conflicts automatically, but these principles will help you make better choices from the start.
Tropical vs. coldwater fish
Never mix goldfish or koi (which prefer 50–72°F) with tropical species like tetras and gouramis (which need 75–82°F). The temperature overlap is too narrow for either group to thrive long-term.
Fin-nipping species
Tiger barbs, serpae tetras, and black skirt tetras are notorious fin-nippers. Never house them with bettas, angelfish, gouramis, or any fish with long, flowing fins. Larger schools of 10+ reduce but do not eliminate this behaviour.
Predator-prey dynamics
If a fish fits in another fish’s mouth, it will eventually end up there. Oscars, large cichlids, and puffers will eat any fish small enough to swallow. Angelfish will eat neon tetras in the wild — and will in your tank too.
Territorial species
Many cichlids, bettas, and some loaches are highly territorial. Provide ample hiding spots, visual breaks, and space to establish territories. Cramped conditions amplify aggression in all species.
Always research a fish’s adult size before purchasing. Many fish sold in shops are juveniles — a 2″ oscar will grow to 14″ within a year and will need a 75-gallon tank to itself. A common pleco sold at 3″ will reach 18″ and produce enormous bioload.
Signs Your Tank May Be Overstocked
Even with a calculator, real-world conditions vary. Test your water regularly and watch for these early warning signs that your stocking level is too high:
| Warning sign | Likely cause | Action |
|---|---|---|
| Ammonia or nitrite above 0 ppm | Filter overwhelmed by bioload | Immediate partial water change; reduce feeding; review stocking |
| Fish gasping at the surface | Low dissolved oxygen | Add surface agitation; check temperature; reduce bioload |
| Frequent fin damage / nipping | Stress from overcrowding | Add more hiding spots; rehome aggressive fish |
| Cloudy water between changes | Bacterial bloom from excess waste | Reduce feeding; increase water change frequency |
| Fish hiding more than usual | Chronic stress or water quality | Test water parameters; assess tank hierarchy |
| Nitrate above 40 ppm weekly | High bioload / insufficient water changes | Increase water change volume or frequency; add live plants |
Frequently Asked Questions
Everything you need to know about stocking your aquarium safely.
How many fish can I put in a 10-gallon tank?
What is the 1-inch-per-gallon rule and is it accurate?
What is bioload in a fish tank?
Can I keep a betta with other fish?
What happens if a fish tank is overstocked?
How does filtration affect how many fish I can keep?
What fish are best for a 55-gallon community tank?
Do schooling fish need to be kept in groups?
Can I mix freshwater and saltwater fish?
How often should I add new fish to avoid stocking problems?
Also on This Site
Not sure how many gallons your tank holds? Our Aquarium Tank Volume Calculator calculates exact water volume for seven tank shapes — rectangular, bow front, cylinder, sphere, hexagon, corner pentagon, and L-shape — accounting for glass thickness, substrate depth, fill level, and a structural safety factor.
Use the volume calculator first to determine your true tank capacity, then return here to plan your stocking list with confidence.