Fish Compatibility Calculator

What Is a Fish Compatibility Calculator?

A fish compatibility calculator helps aquarium hobbyists determine whether different species can peacefully coexist in the same tank. It considers several critical factors: temperament (aggressive vs. peaceful), adult size, water parameter requirements (pH, temperature, hardness), social behavior (schooling, territorial, or solitary), and swimming zone preferences. A community tank with incompatible fish will lead to chronic stress, injuries, disease outbreaks, and even death. This tool gives you a science‑based, hobbyist‑proven compatibility score to avoid heartbreaking mistakes.

While no calculator can guarantee success, our system aggregates knowledge from decades of community aquarium experience, including data from trusted sources such as FishBase, LiveAquaria, SeriouslyFish, and the American Cichlid Association to provide reliable guidance.

Why Fish Compatibility Matters – For Health and Harmony

Key Factors That Determine Fish Compatibility

• Temperament: Fish are broadly classified as peaceful, semi‑aggressive, or aggressive. Peaceful tetras should never be housed with aggressive cichlids. Semi‑aggressive species like angelfish may work in a community if tank space and hiding spots are adequate.
• Adult Size & Growth Rate: A 1‑inch juvenile might be peaceful, but as an adult it could become a predator. Always research maximum size. The size difference between two fish should never exceed 2‑3 inches, and the smaller species must not be small enough to be eaten.
• Water Parameter Requirements: Temperature, pH, GH (general hardness), and KH (carbonate hardness) must overlap significantly. For example, discus require warm, soft, acidic water (82‑86°F, pH 5.5‑6.5), while goldfish prefer cool, hard, alkaline conditions (65‑72°F, pH 7.0‑8.0). They can never be compatible.
• Swimming Zone & Activity Level: Fish occupy top, middle, bottom, or all zones. Overcrowding one zone causes aggression. Highly active danios can stress slow‑moving gouramis even if both are peaceful.
• Social Structure: Schooling fish (neon tetras, corydoras) need groups of at least 6 to feel secure. Territorial species (many cichlids) require space and visual barriers. Solitary bettas often fight with conspecifics and similarly flashy fish.
• Dietary Needs: Carnivores, herbivores, and omnivores differ in feeding behavior. A predator fish housed with small herbivores will see them as food, not tankmates.

Our calculator evaluates each of these factors and provides a compatibility score based on a proprietary algorithm that aggregates data from authoritative sources, including FishBase, the American Cichlid Association, SeriouslyFish, and LiveAquaria profiles.

How to Interpret the Compatibility Score

Popular Community Tank Combinations That Work

Common Incompatible Combinations to Avoid

• Betta with Male Guppies or Other Bettas: Male bettas are aggressive to any fish with long, flowing fins that resemble rival bettas — guppies, endlers, and even fancy goldfish are at high risk. Never keep two male bettas together.
• Angelfish with Neon Tetras: Adult angelfish instinctively prey on small tetras. Neons are a natural food source in the wild. Always keep angelfish with larger tetras (e.g., Black Skirt, Buenos Aires) or fast schooling fish.
• Tiger Barbs with Long‑Finned Fish: Tiger barbs are notorious fin nippers. They will shred the fins of angelfish, bettas, gouramis, and fancy guppies. Keep tiger barbs only with other fast, short‑finned species in large groups of 8+ to minimize aggression.
• Goldfish with Tropical Fish: Goldfish require cold water (65‑72°F), while most tropicals need 75‑82°F. This temperature gap is lethal to one of them. Also, goldfish produce heavy waste, raising ammonia levels that stress tropicals.
• African Cichlids with South American Cichlids (or Tetras): African rift lake cichlids require hard, alkaline water (pH 7.8‑8.6), while Amazonian species need soft, acidic water (pH 6.0‑7.0). They cannot share the same environment safely.
• Oscars with Small Community Fish: Oscars grow to 12‑14 inches and are opportunistic predators. Any fish that fits in their mouth becomes food — including tetras, guppies, and small catfish.

How to Use the Compatibility Calculator Effectively

1. Build your potential community list: Research species that appeal to you. Note their adult size, temperament, and water requirements.
2. Use the calculator as a first pass: Enter species pairs to quickly rule out obvious incompatibilities (e.g., temperature, pH, size mismatch).
3. Check swimming zones: Ensure you have top, middle, and bottom‑dwelling fish to reduce competition for space. The calculator highlights this.
4. Read the detailed considerations: Our result panel provides specific warnings and tips, such as “Neon tetras should be kept in schools of 6+.”
5. Combine with other tools: After confirming compatibility, use our Fish Tank Stocking Calculator to ensure your tank isn't overcrowded.
6. Always have a backup plan: Even compatible fish can occasionally conflict. Prepare a quarantine tank or plan for rehoming if aggression appears after introduction.

Special Cases: Betta, Goldfish & Cichlids

• Betta (Siamese Fighting Fish): Male bettas can sometimes be kept in community tanks with peaceful, non‑nippy fish like harlequin rasboras, corydoras catfish, and neon tetras. Avoid fin‑nippers and other labyrinth fish. Female bettas can be kept together in sororities of 5+ with ample space.
• Goldfish: Goldfish are best kept with other goldfish or hardy cold‑water fish like white cloud mountain minnows and dojo loaches. Never mix fancy goldfish with common goldfish (different swimming speeds). Avoid tropicals — the temperature requirements conflict.
• Cichlids (African & South American): African cichlids from Lake Malawi and Lake Tanganyika require hard, alkaline water and are aggressive. Overstocking and rocky d scaping reduce aggression. South American cichlids (angelfish, discus, rams) need soft, acidic water and more peaceful tankmates. Never mix these two groups.

Frequently Asked Questions

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