Compatible Fish: What Can Live Together in Your Tank

One of the biggest mistakes beginner aquarists make is putting incompatible fish together. Best case: constant stress. Worst case: you find half-eaten fish.

Here's how to build a peaceful community tank where everyone gets along. The Merck Veterinary Manual offers detailed guidance on fish species requirements and aquarium management.

Understanding Fish Compatibility

Factors That Affect Compatibility

Temperament: Aggressive, semi-aggressive, or peaceful

Water parameters: Temperature, pH, hardness requirements must overlap

Size: Fish eat what fits in their mouths

Swimming level: Top, middle, or bottom dwellers

Territory: Some fish are territorial; space matters

Diet: Carnivores may eat their neighbors

Activity level: Hyper fish stress calm fish

Categories of Compatibility

Peaceful Community Fish

Play well with almost everyone:

• Tetras (most species)
• Rasboras
• Corydoras catfish
• Guppies
• Platies
• Mollies (mostly)
• Otocinclus
• Bristlenose plecos
• Harlequin rasboras
• Endlers livebearers

Semi-Aggressive Fish

Okay with some tank mates, but need consideration:

• Gouramis
• Barbs (some species)
• Rainbowfish
• Angelfish
• Mollies (can be nippy)
• Swordtails
• Larger tetras (Buenos Aires)

Aggressive Fish

Usually best alone or in species tanks:

• Cichlids (most species)
• Bettas (males especially)
• Pea puffers
• Oscars
• Jack Dempseys
• Flowerhorns
• Red tail sharks (territorial)
• Chinese algae eaters (aggressive as adults)

Classic Beginner Community Tank

The Setup (20-30 Gallon)

A balanced community might include:

Top level:

• 6 harlequin rasboras

Middle level:

• 6 neon tetras

Bottom level:

• 4-6 corydoras catfish

Cleanup crew:

• 3-4 amano shrimp
• 2-3 nerite snails

This creates activity at all levels with compatible species.

Popular Community Combinations

Peaceful Tropical Community (20-30 gallon)

• 8-10 neon tetras
• 6 corydoras
• 1 bristlenose pleco
• 6 harlequin rasboras
• Nerite snails

Livebearer Community (20 gallon)

• 3 platies
• 3 mollies
• 4 guppies (all males, or expect babies)
• 6 corydoras

Warning: Livebearers breed rapidly. Plan for fry or keep single-sex groups.

Gourami Community (30-40 gallon)

• 1 pair pearl gouramis
• 8 harlequin rasboras
• 6 kuhli loaches
• 6 ember tetras

Angelfish Community (55+ gallon)

• 2-4 angelfish
• 8 rummy nose tetras
• 8 corydoras
• 1 bristlenose pleco

Important: Avoid small fish once angelfish mature—they may become snacks.

Fish to Avoid Combining

Classic Bad Pairings

Bettas + Guppies: Male guppies' colorful fins trigger betta aggression

Bettas + other Bettas: Males will fight to the death

Oscars + small fish: Oscars eat anything that fits in their mouth

Goldfish + tropical fish: Different temperature requirements

Cichlids + peaceful community fish: Most cichlids are too aggressive

Chinese algae eaters + slow fish: CAEs become aggressive and attack tank mates

Neon tetras + angelfish: Once angelfish grow, neons become food

Fin nippers + long-finned fish: Tiger barbs + bettas/guppies = shredded fins

Red Flags

Size difference too large: Big fish eat small fish

Temperature needs don't overlap: Tropical + cold water = someone suffers

One territorial + one timid: Timid fish stress to death

Too many males: Most species need female majority

Schooling Fish Requirements

Many fish MUST be kept in groups:

Single schooling fish become stressed and may die early.

The Swimming Level Rule

Spread fish across all levels for visual interest and reduced conflict:

Top dwellers:

• Hatchetfish
• Guppies
• Endlers
• Danios

Middle dwellers:

• Tetras
• Rasboras
• Mollies
• Platies
• Gouramis
• Angels

Bottom dwellers:

• Corydoras
• Kuhli loaches
• Plecos
• Otocinclus
• Shrimp

Special Considerations

Shrimp and Snails

Usually safe with:

• Small peaceful fish (most won't bother adults)
• Otocinclus
• Small corydoras

Usually eaten by:

• Larger fish
• Pea puffers
• Most cichlids
• Bettas (may eat shrimp)

Baby shrimp are vulnerable: Almost all fish will eat shrimplets

Bettas in Community Tanks

Can bettas live in community? Sometimes.

May work with:

• Corydoras
• Snails
• Ember tetras
• Harlequin rasboras
• Otocinclus

Usually doesn't work with:

• Other bettas (never)
• Guppies (fins trigger aggression)
• Fin nippers
• Fast/active fish (stress bettas)
• Small shrimp (may be eaten)

Individual personality matters. Some bettas are peaceful; some attack everything.

Goldfish

Goldfish are NOT compatible with tropical fish:

• Different temperature requirements
• Goldfish produce massive waste
• Goldfish may eat small tank mates
• Fin nippers shred goldfish fins

Goldfish do well with:

• Other goldfish (similar types)
• White Cloud Mountain Minnows
• Weather loaches
• Large snails

Stocking Levels

Even compatible fish need adequate space.

General Rule

1 inch of fish per gallon is outdated but gives rough guidance.

Better approach:

• Research each species' space needs
• Consider adult size (not purchase size)
• Account for bioload (waste production)
• Use stocking calculators (aqadvisor.com)

Overstocking Signs

• Constant aggression
• Fish gasping at surface
• Ammonia spikes between maintenance
• Stressed/hiding fish
• Disease outbreaks

Before You Buy

Research Checklist

For each fish, know:

• Adult size
• Temperature range
• pH range
• Temperament
• Minimum school size
• Tank size requirements
• Diet (carnivore, herbivore, omnivore)
• Compatibility with current stock

At the Store

• Ask staff about compatibility (but verify independently)
• Observe fish behavior before purchase
• Check for signs of illness
• Don't buy on impulse

The Bottom Line

Successful community tanks require:

1. Research before buying (not after)
2. Matching water parameters across all species
3. Similar temperaments (no mixing aggressive and peaceful)
4. Adequate space for adult sizes
5. Proper school sizes for schooling species
6. Spread across swimming levels for reduced conflict

A well-planned community tank is a joy. A poorly planned one is an ongoing disaster. Plan first, buy second.

Related: How to Set Up Your First Aquarium Related: Best Freshwater Fish for Beginners

Frequently Asked Questions

How do I know if my fish are compatible before adding them together?

Research each species' temperament (peaceful, semi-aggressive, aggressive), water parameter requirements (temperature, pH, hardness), adult size, and preferred swimming level. Compatible fish should overlap in water parameters, differ in aggression level (avoid mixing aggressive species), and not be small enough for larger tank mates to eat. A good rule of thumb: if a fish fits in another fish's mouth, it will eventually end up there.

Can I keep different species of fish with a betta?

Male bettas can coexist with certain peaceful species in tanks of 10+ gallons. Good tank mates include Corydoras catfish, small rasboras, snails, and shrimp. Avoid fin-nipping species like tiger barbs and anything flashy or long-finned that the betta might see as a rival. Always have a backup plan to separate fish if aggression develops, and add the betta last so it doesn't claim the entire tank as territory.

What should I do if my fish are fighting?

Separate the aggressor immediately using a tank divider or by moving them to a different tank. Fighting causes stress, injury, and disease susceptibility in all fish involved. Identify the cause—overcrowding, incompatible species, insufficient hiding spots, or territorial disputes during feeding. Rearranging tank decorations can reset territorial boundaries, but persistent aggression means the fish are truly incompatible and must be permanently separated.