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.
Learn more
- 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.
- Starting a saltwater aquariumAn honest beginner guide to your first marine tank.
- 7 stocking mistakes beginners makeThe avoidable errors that crash a new tank in the first month.
- How to cycle a new aquariumWhy you should never add fish to a brand-new tank on day one.
- 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.