Setting up a small aquarium is one of the most rewarding things you can do as a pet owner. A well-planted tank with colorful fish is calming, beautiful and surprisingly easy to maintain once you understand the basics.
In this guide we cover the best fish species for small aquariums, essential setup tips and the most common mistakes beginners make.
What Is Considered a Small Aquarium?
A small aquarium is generally considered to be between 10 and 60 liters (2.5 to 15 gallons). These tanks are perfect for beginners — they are affordable, easy to maintain and fit comfortably in apartments and small spaces.
Important rule: never overcrowd a small tank. The general guideline is 1 cm of fish per 2 liters of water. Overcrowding causes stress, disease and poor water quality.
Best Fish for Small Aquariums
1. Betta Fish (Siamese Fighting Fish)
The Betta is the most popular small aquarium fish in the world — and for good reason. They are strikingly beautiful, hardy and do not require a filter or heater in warm climates. Males must be kept alone or with non-aggressive tankmates.
- Tank size: minimum 10 liters
- Temperature: 24°C to 28°C
- Difficulty: Very easy
- Compatible with: Snails, shrimp, female bettas (with caution)
2. Neon Tetra
The Neon Tetra is a small, peaceful schooling fish with a brilliant blue and red stripe. They are best kept in groups of at least 6 — a school of Neon Tetras in a planted tank is one of the most beautiful sights in the aquarium hobby.
- Tank size: minimum 40 liters for a school of 6
- Temperature: 22°C to 26°C
- Difficulty: Easy
- Compatible with: Other small peaceful fish
3. Guppy
Guppies are colorful, active and incredibly hardy — making them perfect for beginners. They breed easily in captivity, so consider keeping only males to avoid overpopulation. Available in dozens of color variations.
- Tank size: minimum 20 liters
- Temperature: 22°C to 28°C
- Difficulty: Very easy
- Compatible with: Most small peaceful fish
4. Corydoras Catfish
Corydoras are small bottom-dwelling catfish that spend their time cleaning leftover food from the substrate. They are peaceful, social and best kept in groups of 4 or more. An excellent addition to any community tank.
- Tank size: minimum 40 liters
- Temperature: 22°C to 26°C
- Difficulty: Easy
- Compatible with: Almost all peaceful fish
5. Zebra Danio
Zebra Danios are fast, active and extremely hardy — they can tolerate a wide range of water conditions, making them ideal for cycling a new tank. They are best kept in schools of 6 or more and add constant movement to the aquarium.
- Tank size: minimum 40 liters
- Temperature: 18°C to 26°C
- Difficulty: Very easy
- Compatible with: Most community fish
6. Endler's Livebearer
Endlers are similar to Guppies but smaller and even more colorful. They are peaceful, active and breed readily in captivity. Their small size makes them perfect for nano tanks under 20 liters.
- Tank size: minimum 10 liters
- Temperature: 24°C to 28°C
- Difficulty: Very easy
- Compatible with: Small peaceful fish and shrimp
7. Cherry Shrimp
Technically not a fish, but Cherry Shrimp are one of the best additions to any small aquarium. They are excellent algae cleaners, completely peaceful and fascinating to watch. A colony of red Cherry Shrimp in a planted tank is stunning.
- Tank size: minimum 10 liters
- Temperature: 18°C to 28°C
- Difficulty: Easy
- Compatible with: Small peaceful fish, snails
Essential Equipment for a Small Aquarium
- Filter — essential for maintaining water quality. Use a sponge filter for tanks under 40 liters.
- Heater — required for tropical fish. Set between 24°C and 26°C for most species.
- Light — necessary for live plants and to display your fish's colors. 8 to 10 hours per day.
- Thermometer — monitor water temperature daily.
- Water conditioner — removes chlorine from tap water before adding it to the tank.
- Test kit — monitor ammonia, nitrite, nitrate and pH levels weekly.
The Nitrogen Cycle — The Most Important Concept for Beginners
The biggest mistake beginners make is adding fish to a new tank immediately. A new aquarium must first complete the nitrogen cycle — a process that establishes beneficial bacteria that convert toxic ammonia from fish waste into less harmful compounds.
The cycle typically takes 4 to 6 weeks. You can speed it up by adding a commercial bacterial starter available at any pet store. Never skip this step — it is the number one cause of fish death in new aquariums.
Peces para acuario pequeño
Los mejores peces para acuarios pequeños son el Betta, el Neón, el Guppy, el Danio Cebra y el Corydoras. Estos son resistentes, coloridos y fáciles de cuidar para principiantes. Es fundamental establecer el ciclo del nitrógeno antes de añadir peces al acuario para evitar muertes por acumulación de amoníaco.
El tamaño mínimo recomendado para un acuario comunitario es de 40 litros. Para un Betta solitario, 10 litros son suficientes.
Frequently Asked Questions
How many fish can I put in a small aquarium?
The general rule is 1 cm of fish per 2 liters of water. For a 40-liter tank, you can keep approximately 20 cm of fish — for example, a school of 6 Neon Tetras (3 cm each = 18 cm total) plus a few Corydoras.
How often should I change the water?
Change 20% to 30% of the water every week. Use a gravel vacuum to remove waste from the substrate during water changes. Always treat new water with conditioner before adding it to the tank.
Can I keep a Betta with other fish?
Yes, with care. Bettas can coexist with peaceful bottom dwellers like Corydoras and small snails or shrimp. Avoid fin-nipping species like Tiger Barbs and never keep two male Bettas together.
What is the easiest fish to keep for a beginner?
The Betta fish is the easiest — it tolerates a wide range of water conditions, does not require a school and adapts well to smaller tanks. Guppies and Zebra Danios are close seconds for community tank beginners.
Recommended Products for Small Aquariums
Everything you need to set up your first aquarium:
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Pawpedia Editorial Team
Reviewed against ASPCA and AKC veterinary guidelines · Updated May 2026
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