Fish That FitFree Aquarium Stocking Calculator
Enter your tank size and the fish you have or want. Get an instant bioload percentage, compatibility warnings, and a shortlist of fish that would still fit, for freshwater and saltwater tanks.
Net water volume, not the box size.
Stronger filtration supports a bit more bioload.
Fish in your tank
No fish added yet. Search and add species below to see your bioload and compatibility.
Add fish
- Neon TetraParacheirodon innesi4 cm adult38 L minpeacefulgroup of 6+
- Cardinal TetraParacheirodon axelrodi5 cm adult57 L minpeacefulgroup of 6+
- Ember TetraHyphessobrycon amandae2 cm adult38 L minpeacefulgroup of 8+
- Serpae TetraHyphessobrycon eques4 cm adult76 L minsemi-aggressivegroup of 8+
- Black Skirt TetraGymnocorymbus ternetzi6 cm adult76 L minsemi-aggressivegroup of 6+
- Rummy-nose TetraHemigrammus rhodostomus5 cm adult76 L minpeacefulgroup of 8+
- Harlequin RasboraTrigonostigma heteromorpha4 cm adult38 L minpeacefulgroup of 6+
- Chili RasboraBoraras brigittae2 cm adult19 L minpeacefulgroup of 10+
- Zebra DanioDanio rerio5 cm adult38 L minpeacefulgroup of 6+
- White Cloud Mountain MinnowTanichthys albonubes4 cm adult38 L minpeacefulgroup of 6+
- GuppyPoecilia reticulata5 cm adult38 L minpeacefulgroup of 3+
- Endler's LivebearerPoecilia wingei4 cm adult38 L minpeacefulgroup of 3+
- PlatyXiphophorus maculatus6 cm adult38 L minpeacefulgroup of 3+
- MollyPoecilia sphenops10 cm adult76 L minpeacefulgroup of 3+
- SwordtailXiphophorus hellerii14 cm adult76 L minsemi-aggressivegroup of 3+
- Betta (male)Betta splendens6 cm adult19 L minaggressive
- Dwarf GouramiTrichogaster lalius9 cm adult38 L minpeaceful
- Pearl GouramiTrichopodus leerii11 cm adult114 L minpeaceful
- Honey GouramiTrichogaster chuna5 cm adult38 L minpeaceful
- AngelfishPterophyllum scalare15 cm adult110 L minsemi-aggressive
- German Blue RamMikrogeophagus ramirezi6 cm adult76 L minpeaceful
- Bolivian RamMikrogeophagus altispinosus9 cm adult114 L minpeaceful
- KribensisPelvicachromis pulcher9 cm adult76 L minsemi-aggressive
- Electric Yellow CichlidLabidochromis caeruleus10 cm adult151 L minsemi-aggressive
- OscarAstronotus ocellatus30 cm adult284 L minaggressive
- Convict CichlidAmatitlania nigrofasciata13 cm adult114 L minaggressive
- Corydoras CatfishCorydoras sp.6 cm adult76 L minpeacefulgroup of 6+
- Pygmy CorydorasCorydoras pygmaeus3 cm adult38 L minpeacefulgroup of 8+
- Bristlenose PlecoAncistrus sp.13 cm adult114 L minpeaceful
- Common PlecoHypostomus plecostomus46 cm adult473 L minpeaceful
- Kuhli LoachPangio kuhlii10 cm adult76 L minpeacefulgroup of 5+
- Otocinclus CatfishOtocinclus sp.4 cm adult38 L minpeacefulgroup of 6+
- Cherry BarbPuntius titteya5 cm adult57 L minpeacefulgroup of 6+
- Tiger BarbPuntigrus tetrazona7 cm adult76 L minsemi-aggressivegroup of 8+
- Boesemani RainbowfishMelanotaenia boesemani10 cm adult114 L minpeacefulgroup of 6+
- Gold BarbBarbodes semifasciolatus8 cm adult76 L minpeacefulgroup of 6+
- Cherry ShrimpNeocaridina davidi4 cm adult19 L minpeacefulgroup of 10+
- Amano ShrimpCaridina multidentata5 cm adult38 L minpeacefulgroup of 3+
- Nerite SnailNeritina sp.3 cm adult19 L minpeaceful
- Glowlight TetraHemigrammus erythrozonus4 cm adult38 L minpeacefulgroup of 6+
- Lemon TetraHyphessobrycon pulchripinnis5 cm adult76 L minpeacefulgroup of 6+
- Black Neon TetraHyphessobrycon herbertaxelrodi4 cm adult57 L minpeacefulgroup of 6+
- Congo TetraPhenacogrammus interruptus9 cm adult114 L minpeacefulgroup of 6+
- Celestial Pearl DanioDanio margaritatus3 cm adult38 L minpeacefulgroup of 6+
- Rosy BarbPethia conchonius8 cm adult114 L minsemi-aggressivegroup of 6+
- Denison BarbSahyadria denisonii11 cm adult208 L minpeacefulgroup of 6+
- Clown LoachChromobotia macracanthus30 cm adult473 L minpeacefulgroup of 5+
- Sparkling GouramiTrichopsis pumila4 cm adult38 L minpeaceful
- Paradise FishMacropodus opercularis10 cm adult76 L minaggressive
- Cockatoo Dwarf CichlidApistogramma cacatuoides8 cm adult76 L minsemi-aggressive
- Firemouth CichlidThorichthys meeki15 cm adult114 L minsemi-aggressive
- Jack DempseyRocio octofasciata20 cm adult208 L minaggressive
- DiscusSymphysodon aequifasciatus15 cm adult208 L minpeacefulgroup of 5+
- Rainbow SharkEpalzeorhynchos frenatum15 cm adult189 L minsemi-aggressive
- Mystery SnailPomacea bridgesii5 cm adult19 L minpeaceful
Add the fish you have or want to see your bioload and any compatibility issues.
Fish that would still fit
Compatible species with room to spare. Click to add a group.
How the stocking calculator works
- 1
Set up your tank
Choose freshwater or saltwater, switch units between US gallons and litres, enter your tank volume, and pick your filtration level.
- 2
Add your fish
Search the built-in species list and add the fish you keep or want. Schooling fish are added as a proper group.
- 3
Read the results
See your live bioload percentage, plain-English compatibility warnings, and a shortlist of fish that still fit.
What Fish That Fit checks
Bioload & capacity
Each species is weighted by its real adult waste output, not a crude inch-per-gallon rule, and balanced against your tank size and filtration.
Temperament & aggression
Flags aggressive fish housed with peaceful ones, plus fin-nippers paired with slow, long-finned tankmates like bettas and angelfish.
Schooling & group size
Warns when shoaling species such as neon tetras or cory catfish are kept in groups too small to feel secure.
Water parameters
Checks that your fish share a workable temperature and pH window, so you do not mix coldwater and tropical species by accident.
Predator & prey
Highlights when a fish is big enough to eat its tankmates, or your cleanup shrimp, before it happens in the tank.
Minimum tank size
Catches fish that are sold small but grow large, like common plecos and oscars, that simply outgrow a starter tank.
Why stocking the right way matters
Most aquarium problems trace back to a single mistake: too many fish, too soon, in a tank that is too small. An overstocked tank produces more ammonia than the filter bacteria can process, water quality swings, and fish get stressed, which is when disease takes hold. Getting your bioload right is the difference between a tank you fight with and one that mostly runs itself.
Stocking is also about temperament, not just numbers. A peaceful community of tetras and corydoras falls apart the moment a fin-nipping tiger barb or a hungry angelfish joins it. Fish That Fit weighs adult size, waste output, schooling needs, aggression, and water parameters together, the same things an experienced aquarist checks before buying a fish.
Why the inch-per-gallon rule falls short
“One inch of fish per gallon” is the first rule most beginners learn, and it is also the one that gets them into trouble. It treats a ten-inch pleco the same as ten one-inch tetras, when the pleco produces far more waste, needs far more swimming room, and will eventually outgrow the tank entirely. A proper aquarium stocking calculator looks at adult size, real waste output, and behaviour together, not a single number that ignores all three.
How a bioload calculator reads your tank
Every fish you add raises the ammonia your filter has to process. Fish That Fit's bioload calculator weighs each species by its adult size and waste output, balances that against your tank volume and filtration strength, and gives you a single percentage. Staying under about 85% leaves a comfortable margin for the swings that come with feeding, water changes, and the occasional skipped maintenance day.
What a fish compatibility checker actually checks
Compatibility is more than “will they fight.” Our fish compatibility checker flags aggressive fish housed with peaceful tankmates, fin-nippers paired with long-finned fish like bettas and angelfish, predators large enough to eat their neighbours (or your cleanup shrimp), and species that need different temperatures or pH ranges than the rest of your community. Our fish compatibility guide explains all five checks in plain English.
Schooling fish need numbers, not just space
Neon tetras, corydoras, and rasboras are shoaling species: kept in groups that are too small, they spend their time stressed and hiding rather than behaving naturally. The calculator checks each schooling species against its real minimum group size, so a tank that looks fine on paper does not quietly become an unhappy one in practice.
Learn more
Browse every aquarium stocking and fish care article on the blog, or work through our step-by-step setup and stocking guides. A good place to start is how to use the stocking calculator. Or jump straight into a popular guide below:
- How to set up a freshwater aquariumA complete, in-order walkthrough from empty tank to first fish.
- How to cycle a new aquariumWhy you should never add fish to a brand-new tank on day one.
- Fish compatibility explainedThe five things that actually decide whether two fish can share a tank.
- The nitrogen cycle explainedWhere the beneficial bacteria really live, and the mistakes that crash them.
- How many fish in a 5-gallon tank?What genuinely suits a nano tank, and why "small" means thinking in ones, not crowds.
- How many fish in a 10-gallon tank?Realistic stocking ideas for the most popular beginner tank size.
- How many fish in a 20-gallon tank?Five community setups for the beginner sweet-spot tank.
- How many fish in a 29-gallon tank?Five stocking plans for the tall, in-between size that rewards a bit of restraint.
- How many fish in a 40-gallon breeder?Why the wide footprint beats a taller tank, with five stocking plans.
- How many fish in a 55-gallon tank?Big schools, centrepiece fish, and how to use the extra space well.
- How many fish in a 75-gallon tank?Where stocking gets ambitious: real centrepiece fish and serious cichlid setups.
- How often to do a water changeHow often, how much, and why a weekly habit beats a monthly rescue.
- Water parameters explainedAmmonia, nitrite, nitrate, pH, and hardness in plain English.
- What is bioload?A plain-English explainer on the number this calculator is built around.
- Is your tank overstocked?The warning signs, the real risks, and a fix ladder that starts with a bucket.
- Beating aquarium algaeWhy you have it, how to identify the type, and the fix that works for each.
- How to choose an aquarium filterSponge, hang-on-back, or canister? How to pick and size one.
- Why are my fish dying?Work through the real causes in order, from water quality to tankmates.
- Is the "1 inch per gallon" rule true?Where the rule comes from, why it breaks down, and what to use instead.
- 7 stocking mistakes beginners makeThe avoidable errors that crash a new tank in the first month.
- 12 best beginner freshwater fishHardy, affordable species that forgive the mistakes every new keeper makes.
- Betta fish careThe real minimum tank size, the heater you need, and the bowl myth retired.
- How many goldfish can you keep?Why goldfish need far more room than a bowl, and how many actually fit.
- 8 easy aquarium plants for beginnersHardy, low-light plants that need no CO2 and forgive neglect.
- Starting a saltwater aquariumAn honest beginner guide to your first marine tank.
- 10 best beginner saltwater fishHardy, reef-safe marine fish that forgive a new keeper.
- Using Fish That FitA two-minute walkthrough of every input and what the results mean.
Frequently asked questions
- How many fish can I put in my aquarium?
- It depends on the adult size of the fish, your tank volume, and your filtration, not just a simple inch-per-gallon rule. Enter your tank size and the fish you want and Fish That Fit gives you a bioload percentage, where under about 85% leaves a comfortable safety margin for a beginner.
- Is the "1 inch of fish per gallon" rule accurate?
- It is a rough starting point at best and breaks down quickly. A 10-inch pleco produces far more waste than ten 1-inch neon tetras, and the rule ignores swimming room, territory, and adult size. This calculator weights each species by its real waste load instead.
- What is bioload?
- Bioload is the total biological waste your livestock produces, mostly ammonia from respiration and food. Your filter and beneficial bacteria have to process all of it. The higher your bioload relative to the tank, the harder it is to keep water parameters stable.
- Does this work for saltwater tanks?
- Yes. Switch the water type to saltwater and you get reef-appropriate species, marine compatibility checks, and a more conservative bioload target, since reef tanks are less forgiving than freshwater community tanks.
- Why does it warn me about fish I already own?
- Fish That Fit flags common problems: aggressive fish housed with peaceful ones, fin-nippers kept with long-finned fish, schooling fish in groups that are too small, predators that can eat their tankmates, and species that need different temperatures or more space than your tank offers.
- Should I add all my fish at once?
- No. Stock slowly so your filter bacteria can catch up with the rising bioload. Add a few fish, wait two to three weeks while testing for ammonia and nitrite, then add the next group. A brand-new tank should be fully cycled first.
- Do you store the data I enter?
- No. Every calculation runs locally in your browser using a built-in dataset. Nothing you type is sent to a server or saved anywhere.