Freshwater Community Fish Compatibility Guide
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Full bio →Before stocking, use the fish tank stocking calculator to confirm your tank has capacity. Then use this guide to verify each species will coexist peacefully with the others.
The Three Compatibility Factors
1. Aggression level — Does the fish attack or stress tank mates?
2. Water parameters — Do the fish share temperature and pH preferences?
3. Size ratio — Can the larger fish eat the smaller fish?
All three must align for a healthy community. A peaceful fish kept at the wrong temperature or with a fish that can swallow it is not a successful community.
Aggression Level Chart
| Level | Description | Examples |
|---|---|---|
| Peaceful | Ignores most tank mates | Neon tetra, corydoras, rasboras, otocinclus |
| Semi-aggressive | May chase similar-looking fish or fin-nippers | Tiger barb, serpae tetra, angelfish |
| Aggressive | Attacks smaller fish, territorial | Betta (male), oscar, jack dempsey, red devil |
| Species-only | Cannot be kept with any other fish | Betta male with another male betta |
Safe Community Combinations (Peaceful + Peaceful)
These species commonly coexist in 20–55 gallon community tanks:
| Species | Min school size | Temp (°F) | pH | Notes |
|---|---|---|---|---|
| Neon tetra | 6+ | 72–78 | 6.0–7.0 | Keep with similar-sized fish only |
| Cardinal tetra | 6+ | 73–81 | 4.5–7.0 | Softer water than neons |
| Harlequin rasbora | 6+ | 72–79 | 6.0–7.5 | Good community fish |
| Corydoras catfish | 4+ | 70–80 | 6.0–7.5 | Bottom dweller, peaceful |
| Platy | 3+ | 70–80 | 7.0–8.2 | Hardy, good beginner fish |
| Molly | 3+ | 72–82 | 7.5–8.5 | Prefer harder, slightly salty water |
| Otocinclus | 3+ | 72–82 | 6.0–7.5 | Algae eater, very peaceful |
| Dwarf gourami | 1–2 | 72–82 | 6.0–7.5 | Males may fight each other |
| Cherry barb | 6+ | 73–81 | 6.0–7.0 | Peaceful barb, unlike tiger barbs |
Species to Use Carefully
These species need thoughtful placement — not all community tanks suit them:
Tiger barb: Notorious fin-nippers. Will shred the fins of slow-moving or long-finned fish (bettas, angelfish, fancy guppies). Keep in schools of 8+ (reduces nipping behavior) and only with fast, short-finned tank mates.
Angelfish: Peaceful with adults of similar size. Will eat neon tetras and small tetras — despite being sold together in pet stores. A 4-inch angelfish can swallow a 1-inch neon whole.
Betta (male): Can be kept in community tanks with fast, non-nippy, non-colorful tank mates. Incompatible with: other male bettas, gouramis, paradise fish, anything that resembles a betta, or fin-nippy species. Compatible with: corydoras, otocinclus, small rasboras, snails.
Goldfish: Coldwater fish (65–75°F). Incompatible with tropical fish. High bioload. Should only be kept with other goldfish or cold-tolerant species (white cloud minnows, weather loaches).
The “Size Rule” for Predation
A general rule: if a fish can fit the other fish in its mouth, it will eventually try. For fish over 3 inches, ensure tank mates are at least 50% of the predator’s body length.
| Predator | Do not keep with |
|---|---|
| Oscar (10–14 inches) | Anything under 4–5 inches |
| Angelfish (4–6 inches) | Neons, small tetras, tiny rasboras |
| Larger cichlids | Small community fish of any kind |
Water Parameter Compatibility
Fish must share compatible temperature and pH ranges. Mixing fish with incompatible requirements creates chronic stress — one group is always out of its optimal range.
| Type | Temp (°F) | pH |
|---|---|---|
| South American tetras, corydoras, discus | 76–86 | 5.5–7.0 |
| Central American / common tropicals | 72–80 | 6.5–7.5 |
| African rift lake cichlids | 75–82 | 7.8–8.5 |
| Goldfish | 65–72 | 7.0–8.0 |
African rift lake cichlids (mbuna, peacock cichlids) require hard, alkaline water that is incompatible with soft-water tetras or corydoras. Never mix these groups.
Planning a Community: A Simple Framework
- Pick a centerpiece fish (1 species, often the largest or most visually prominent)
- Add a mid-water schooling fish (6–8 fish of the same species, peaceful tetras or rasboras)
- Add bottom dwellers (corydoras or otocinclus for algae control and waste processing)
- Optional: top-dwelling fish or surface feeders
Check all selections share overlapping temperature and pH ranges. Run the final list through the fish tank stocking calculator to confirm the bioload is within your tank’s capacity.
For stocking amounts by tank size, see fish tank size guide. For errors to avoid when building your community, see aquarium stocking mistakes.