Getting your stocking level right is the single most important factor in keeping a healthy aquarium. 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.
The calculator runs entirely in your browser with no sign-up required.
Type your tank size in US gallons or litres. Unsure? Use our tank volume calculator first.
Freshwater, saltwater, and brackish each have distinct fish communities. Tall tanks have less oxygenated surface area than long shallow tanks of the same volume.
Select honestly — overestimating filtration is the most common stocking mistake.
Filter by category or search by name. Click + to add fish and adjust quantities in the Your Tank panel.
Checks for aggression, fin-nipping, temperature conflicts, school minimums, and overstocking. Address red warnings before purchasing.
Configure your tank below, then browse the fish library to build your community. The stocking meter and compatibility panel update in real time.
Add fish to see compatibility notes.
Bioload is the total organic waste that your tank's inhabitants produce. When fish excrete, ammonia enters the water. Beneficial bacteria in your filter convert this to nitrite, then to the less toxic nitrate. If waste production exceeds your filter's 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.
Large body mass, messy eating habits (goldfish, cichlids, oscars), high metabolism, a protein-heavy diet, and infrequent water changes all raise the biological burden on your filter.
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.
Maximum bioload points and example communities for common sizes with standard filtration and rectangular shape.
| Tank size | Stocking capacity | Stocking level | Example community |
|---|---|---|---|
| 5 gallon | Very low | Nano only | 8x Chili Rasbora or 1x Betta |
| 10 gallon | Low | Small community | 8x Neon Tetra + 4x Corydora |
| 20 gallon | Moderate | Community | 8x Cardinal + 6x Corydora + 2x Dwarf Gourami |
| 29 gallon | Moderate | Community | 10x Rummynose + 6x Corydora + 2x Gourami + 1x Bristlenose |
| 40 gallon | Good | Medium community | 10x Rummynose + 6x Corydora + 1 pair Kribensis + 1x Bristlenose |
| 55 gallon | Good | Full community | 10x Cardinal + 8x Harlequin + 6x Corydora + 2x Pearl Gourami + 1x Bristlenose |
| 75 gallon | High | Spacious | 4x Angelfish + 10x Rummynose + 8x Corydora + 6x Clown Loach |
| 125 gallon | Very high | Large display | 6x Discus + 10x Cardinal + 8x Corydora + Planted biotope |
Always aim to stock at 60–75% of your tank's effective capacity to leave headroom for growth, fry, and filter resilience. The stocking meter turns amber at 80%.
A single goldfish produces as much waste as 10 neon tetras despite being only 6x longer -- bioload reflects this reality.
| Species | Adult size | Bioload level | Min. school | Temperament |
|---|---|---|---|---|
| Neon Tetra | 1.5" | Low | 6 | Peaceful |
| Cardinal Tetra | 2" | Low | 6 | Peaceful |
| Ember Tetra | 0.8" | Very low | 8 | Peaceful |
| Congo Tetra | 3.5" | Medium | 6 | Peaceful |
| Black Skirt Tetra | 2.5" | Low | 6 | Fin nipper |
| Chili Rasbora | 0.7" | Very low | 8 | Peaceful |
| Harlequin Rasbora | 2" | Low | 8 | Peaceful |
| Celestial Pearl Danio | 1" | Very low | 8 | Peaceful |
| Zebra Danio | 2" | Low | 6 | Peaceful |
| Guppy | 2" | Low | 3 | Peaceful |
| Platy | 2.5" | Low | 3 | Peaceful |
| Molly | 3.5" | Medium | 3 | Peaceful |
| Corydoras Catfish | 2.5" | Low | 6 | Peaceful |
| Otocinclus | 2" | Low | 4 | Peaceful |
| Bristlenose Pleco | 5" | High | 1 | Peaceful |
| Common Pleco | 18" | Very high | 1 | Large / messy |
| Dwarf Gourami | 3.5" | Medium | 2 | Peaceful |
| Honey Gourami | 2" | Low | 2 | Peaceful |
| Betta | 2.5" | Medium | 1 | Aggressive |
| Angelfish | 6" | High | 2 | Semi-aggressive |
| German Blue Ram | 3" | Medium | 2 | Sensitive |
| Discus | 8" | Very high | 4 | Expert |
| Oscar | 14" | Very high | 1 | Aggressive |
| Tiger Barb | 3" | Medium | 8 | Fin nipper |
| Cherry Barb | 2" | Low | 6 | Peaceful |
| Common Goldfish | 12" | Very high | 2 | Messy / coldwater |
| Clown Loach | 12" | High | 4 | Peaceful |
| Kuhli Loach | 4" | Low | 3 | Peaceful |
| Cherry Shrimp | 1.2" | Very low | 10 | Peaceful |
| Clownfish | 4" | High | 2 | Peaceful |
| Maroon Clownfish | 6" | High | 2 | Aggressive |
| Banggai Cardinalfish | 3" | Medium | 3 | Peaceful |
| Blue Tang | 12" | Very high | 1 | Active |
| Yellow Tang | 8" | High | 1 | Reef safe |
| Purple Tang | 8" | High | 1 | Aggressive to tangs |
| Sailfin Tang | 15" | Very high | 1 | Needs space |
| Tomini Tang | 6" | High | 1 | Reef safe |
| Foxface Rabbitfish | 9" | High | 1 | Peaceful |
| Lawnmower Blenny | 5" | Medium | 1 | Peaceful |
| Midas Blenny | 5" | Medium | 1 | Peaceful |
| Firefish Goby | 3" | Medium | 1 | Peaceful |
| Neon Goby | 2" | Low | 2 | Reef safe |
| Lyretail Anthias | 3.5" | Medium | 5 | Schooling |
| Coral Beauty Angelfish | 4" | Medium | 1 | Semi-aggressive |
| Flame Angelfish | 4" | Medium | 1 | Semi-aggressive |
| Emperor Angelfish | 15" | Very high | 1 | Semi-aggressive |
| Six-Line Wrasse | 3" | Medium | 1 | Semi-aggressive |
| Melanurus Wrasse | 5" | Medium | 1 | Semi-aggressive |
| Copperband Butterflyfish | 8" | High | 1 | Expert |
| Longnose Butterflyfish | 9" | High | 1 | Expert |
| Orchid Dottyback | 3" | Medium | 1 | Aggressive |
| Snowflake Moray Eel | 24" | Very high | 1 | Semi-aggressive |
| Porcupine Puffer | 12" | Very high | 1 | Semi-aggressive |
| Lined Seahorse | 6" | Medium | 2 | Expert |
| Cleaner Shrimp | 2" | Very low | 1 | Peaceful |
| Archerfish | 10" | High | 3 | Brackish |
| Scat | 12" | Very high | 3 | Brackish / semi-aggr. |
| Bumblebee Goby | 1.5" | Low | 4 | Brackish / peaceful |
| Knight Goby | 3" | Medium | 1 | Brackish / peaceful |
| Colombian Shark Catfish | 10" | Very high | 3 | Brackish / predator |
| Figure-8 Puffer | 3" | Medium | 1 | Brackish / aggressive |
| Green Spotted Puffer | 6" | High | 1 | Brackish / aggressive |
| Target Puffer | 6" | High | 1 | Brackish / aggressive |
| Fiddler Crab | 2" | Low | 3 | Brackish / semi-aggr. |
| Red Claw Crab | 2" | Low | 2 | Brackish / semi-aggr. |
Stocking level is only half the puzzle. The compatibility panel above catches the most common conflicts automatically.
Never mix goldfish or koi (50-72F) with tropical species like tetras and gouramis (75-82F). The temperature overlap is too narrow for either group to thrive.
Tiger barbs, serpae tetras, and black skirt tetras are notorious fin-nippers. Never house them with bettas, angelfish, gouramis, or any fish with flowing fins.
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.
Many cichlids, bettas, and some loaches are highly territorial. Provide ample hiding spots and visual breaks. Cramped conditions amplify aggression in all species.
Always research a fish's adult size before purchasing. A 2" oscar will grow to 14" within a year. A common pleco sold at 3" reaches 18" and produces enormous bioload.
Test your water regularly and watch for these early warning signs:
| Warning sign | Likely cause | Action |
|---|---|---|
| Ammonia or nitrite above 0 ppm | Filter overwhelmed | Immediate water change; reduce feeding; review stocking |
| Fish gasping at the surface | Low dissolved oxygen | Add surface agitation; reduce bioload |
| Frequent fin damage | Stress from overcrowding | Add 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 parameters; assess tank hierarchy |
| Nitrate above 40 ppm weekly | High bioload or insufficient changes | Increase water change volume; add live plants |
Everything you need to know about stocking your aquarium safely.
Not sure how many gallons your tank holds? Our Aquarium Tank Volume Calculator calculates exact water volume for seven tank shapes accounting for glass thickness, substrate depth, fill level, and structural safety factor.
Use the volume calculator first to determine your true tank capacity, then return here to plan your stocking list with confidence.