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Stocking and compatibility without shortcuts

A compatible tank is a whole system of adult fish, water, space and behaviour, not a yes-or-no pair of species.

3 min readUpdated October 3, 2026

About this guide

Plan the whole aquarium

By Fishician

Published July 23, 2026

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The short answer

  • Plan for adults. Not juveniles, and not inches per gallon.
  • Check the whole list together. Two species can share water but fight over one cave or see each other as food.
  • Water tests check only part of the plan. Rising ammonia or nitrite calls for investigation. Good readings do not prove there is enough space or compatible behavior.
  • Add in stages. Add only after research and quarantine. Removal can be difficult, so arrange separation before introducing fish.
  • Have a separation plan first. A spare cycled filter, divider or return agreement before you buy.
Guppies, tetras and a corydoras in a community aquarium
A mixed-species aquarium. A photograph of fish together does not establish long-term compatibility.Photo: Comunidad acuario — El fosilmaníaco · CC BY 3.0. Resized for display.
Three checks every plan must pass
Water overlap
  • Temperature and salinity
  • pH, hardness, alkalinity
  • Oxygen and flow for everyone
Space and territory
  • Adult footprint and swim zone
  • Shelters and sight lines
  • Proper group sizes
Contingency
  • Quarantine or divider ready
  • Return or rehome option
  • Known before introduction

Use a conflict ledger, not an average score

Copy one row for each proposed species: Scientific name ___; adult dimensions ___; required group ___; water requirements and source ___; swimming zone ___; territory and shelter ___; diet and feeding access ___; predation or harassment risks ___. Beside it, write the planned aquarium's footprint and the evidence for each requirement.

Hypothetical example: two candidates share a temperature range, but both rely on the same limited bottom territory. Passing the temperature check cannot cancel the space conflict. A third species with unknown adult needs is unresolved, even if a comparison tool shows no warning. Obtain the missing information, change the plan or omit the candidate.

For every conflict, record issue ___; evidence ___; proposed change ___; separation option ___; decision ___. Do not buy an animal on the assumption that aggression will be harmless or that you will find a larger tank later. If fish already show injury or exclusion from food, arrange safe separation and appropriate care promptly.

Build an adult worksheet

  • For each fish: scientific name, adult size, shape, activity zone, water needs, group size, temperament, diet, waste and risks to plants or invertebrates.
  • Common names can mean different animals. Confirm the species.
  • A pairwise checker is a prompt for research, not proof the whole plan works.

Usable space

  • Long swimmers need length; bottom dwellers compete for floor even when upper water looks empty.
  • Rock creates territories but takes swimming room.
  • Give several shelters and feeding spots so one bully can't control them all.

Aggression depends on setup

  • Charts describe tendencies. Footprint, sight breaks, hiding places, group size, feeding and introduction order decide outcomes.
  • Rearranging rock before adding a newcomer reduces residents' home advantage.
  • Watch for quiet aggression: a fish shut out of food, stuck in a corner, or with overnight fin damage. See normal behaviour.

When to say no

  • Core water needs don't overlap, adult space is too small, or predation is credible.
  • Social needs can't be met, or you have no way to separate fish.
  • Evidence is incomplete. Wait. Buying should be the last step.

Bioload is evidence, not a formula

  • Waste varies hugely between fish of the same length, and with diet, feeding, temperature and life stage.
  • What a tank can absorb depends on the biofilter, circulation, oxygen, plants, water changes and trapped debris.
  • Undetectable ammonia and nitrite and manageable nitrate are useful checks, not proof of safe stocking. Space, oxygen, behavior and social needs still matter.
  • Detectable ammonia or nitrite after adding fish needs prompt investigation; stop adding fish while addressing the cause.

Sources

Each source is listed with the part of this guide it supports.