Fish Compatibility Checker

How This Tool Works

This calculator checks compatibility between two fish species based on known compatibility data from aquarium keeping experience and research. It evaluates multiple factors including temperament (peaceful, semi-aggressive, aggressive), size differences, water parameter requirements (pH, temperature, hardness), swimming level preferences, territorial behavior, and dietary needs. The tool provides a compatibility score from 0-100 and a detailed assessment of whether the fish can safely live together.

Fish compatibility is crucial for a healthy, stress-free aquarium. Incompatible fish can lead to aggression, stress, injury, and even death. While this tool provides a good starting point, individual fish personalities can vary, and tank size, decor, and other tankmates also affect compatibility. Always research specific species requirements and monitor fish behavior when introducing new tankmates. A compatibility score above 70 generally indicates good compatibility, while scores below 50 suggest potential issues.

Factors Affecting Fish Compatibility

• Temperament: Peaceful fish should be kept with other peaceful species. Aggressive fish may attack or stress others
• Size: Large fish may eat or intimidate smaller fish. Similar-sized fish are generally more compatible
• Water Parameters: Fish with different pH, temperature, or hardness requirements may not thrive together
• Swimming Level: Fish that occupy different tank levels (top, middle, bottom) are often more compatible
• Territorial Behavior: Territorial fish need adequate space and hiding spots to coexist
• Diet: Some fish are predators and will eat smaller tankmates
• Schooling Behavior: Schooling fish need groups of their own species to feel secure
• Tank Size: Larger tanks can accommodate more diverse fish combinations

Compatibility Best Practices

• Research First: Always research specific species requirements before purchasing
• Quarantine New Fish: Quarantine new fish for 2-4 weeks to prevent disease introduction
• Introduce Gradually: Add new fish gradually, not all at once, to prevent overwhelming the biological filter
• Provide Hiding Spots: Include plants, rocks, and decorations to provide territories and hiding places
• Monitor Behavior: Watch for signs of aggression, stress, or bullying after introducing new fish
• Schooling Fish: Keep schooling fish in groups of 6+ of the same species
• Tank Size Matters: Larger tanks reduce territorial disputes and aggression
• Have a Backup Plan: Be prepared to separate incompatible fish if problems arise

FAQ

• Which fish can live together?
  Compatible fish typically have similar temperaments, size, and water requirements. Peaceful community fish like guppies, platies, tetras, rasboras, and corydoras can often live together in community tanks. Semi-aggressive fish like some barbs and gouramis can coexist with careful selection. Aggressive fish like cichlids should be kept with caution and only with other compatible aggressive species. Always research specific species - even within the same family, compatibility varies. A good rule: if one fish can fit in another's mouth, they're not compatible.
• What affects fish compatibility?
  Multiple factors affect compatibility: Temperament (peaceful vs. aggressive), size (large fish may eat small ones), water parameters (pH, temperature, hardness must be compatible), swimming level (top, middle, bottom dwellers), territorial behavior (some fish need space), dietary needs (predators vs. herbivores), and schooling behavior (some need groups). Tank size also matters - larger tanks can accommodate more diverse combinations. Always research specific species requirements before adding them to your tank.
• What does the compatibility score mean?
  The compatibility score (0-100) indicates how well two fish species can coexist: 80-100: Excellent compatibility, safe to keep together. 60-79: Good compatibility, generally safe with proper tank setup. 40-59: Moderate compatibility, may work but requires careful monitoring and adequate space. 20-39: Poor compatibility, high risk of problems, not recommended. 0-19: Very poor compatibility, should not be kept together. Scores consider multiple factors, but individual fish personalities and tank conditions can affect actual compatibility.
• Can I keep aggressive and peaceful fish together?
  Generally, no. Aggressive fish will stress, injure, or kill peaceful fish. However, some semi-aggressive fish can coexist with peaceful species if: the tank is large enough, there are plenty of hiding spots, the peaceful fish are fast swimmers, and you monitor closely. Some exceptions exist (like keeping certain cichlids with fast-swimming dither fish), but these require experience and careful selection. Beginners should stick to all peaceful or all aggressive setups.
• How important is tank size for compatibility?
  Tank size is crucial for compatibility. Larger tanks reduce territorial disputes, provide escape routes, and allow fish to establish territories. A fish that's compatible in a 200L tank may be aggressive in a 50L tank. Overcrowding increases stress and aggression. Follow the "1 inch of fish per gallon" rule as a minimum, but research specific space requirements for each species. Some fish need more space than others. When in doubt, choose a larger tank.
• What should I do if my fish are fighting?
  If fish are fighting: First, ensure your tank is large enough and has adequate hiding spots. Rearrange decorations to break up territories. Check water parameters - poor water quality increases stress and aggression. Consider adding more of the same species (for schooling fish) or removing the aggressor. If aggression is severe, separate the fish immediately. Some aggression is normal during territory establishment, but persistent chasing, fin nipping, or hiding indicates a serious problem. Have a backup tank or divider ready.