Best Fish Tanks & Aquariums — 21 Litre (2026)
1 fish tanks & aquariums matching capacity “21 Litre”, hand-picked from UK Amazon listings.
Not sure which one to pick? Read our How to Care for & Maintain Your Fish Tanks & Aquariums guide.
Looking for the perfect starter aquarium? Our selection of 21 litre fish tanks provides a fantastic entry point into the world of aquatic keeping. These compact aquariums are ideal for beginners, allowing you to observe and nurture small freshwater fish without requiring a large space or significant investment. They offer a beautiful display piece for any room while providing a suitable environment for colourful shrimp, neon tetras, or other small schooling fish.
Each 21 litre tank is crafted from durable glass, ensuring long-lasting enjoyment. We've curated a range of styles and designs to complement your home décor, from sleek and modern to more traditional looks. Remember to thoroughly research the specific needs of any fish you plan to house within this size aquarium – proper filtration and temperature control are crucial for their health and wellbeing.
These aquariums represent an accessible pathway into responsible fishkeeping. Before purchasing, double-check compatibility with your chosen fish species and ensure you have all necessary equipment such as a heater, filter, and substrate. You can find detailed product information and specifications on Amazon UK listings for each tank in this collection.
#1 Pick
Aqua One Platform 21 Aquarium Fish Tank 40cm 21L, Built-In LED Lighting and Filter System- 21 Litre
£91.00
Pros
- White and blue built-in LEDs cover both daytime viewing and a moonlight setting
- Removable glass lid keeps evaporation down and stops fish jumping out
- Compact 40cm footprint suits a bedroom or office desk
Cons
- Needs a separate heater for anything other than coldwater species
FAQ
What kind of fish are suitable for a 21 litre aquarium?+
Small, peaceful freshwater fish thrive best in a 21-litre tank. Ideal choices include neon tetras, ember tetras, small schooling danios, or cherry shrimp. Avoid larger or more aggressive species that require significantly more space.
What filter do I need for a 21 litre aquarium?+
A small internal filter is typically sufficient for a 21-litre tank. Look for filters rated for tanks up to 30 litres – this provides adequate filtration without overpowering the smaller volume. Ensure you regularly clean and maintain your chosen filter.
What temperature should I keep my 21 litre aquarium at?+
Most tropical fish require a water temperature between 24°C and 28°C. A submersible heater is essential for maintaining this consistent temperature, particularly in cooler UK homes. Monitor the temperature regularly with a thermometer.
What substrate should I use in my 21 litre aquarium?+
Gravel or sand are common substrates used in 21-litre tanks. Choose a size appropriate for your chosen fish – finer gravel is better for smaller species, while larger gravel can be easier to clean. Avoid sharp substrates that could injure delicate fins.
How often should I do water changes in my 21 litre aquarium?+
Regular partial water changes are vital for maintaining a healthy aquarium. Perform a 25-50% water change every week or two, depending on fish load and filtration efficiency. Always use dechlorinated water.
Do I need a lid for my 21 litre aquarium?+
A lid is highly recommended to prevent evaporation and keep debris out of the tank. It also helps maintain stable temperature levels and prevents fish from jumping out. Choose a lid that fits snugly over your aquarium.
What lighting do I need for a 21 litre aquarium?+
LED aquarium lights are an excellent choice for providing illumination and supporting plant growth (if you choose to include live plants). Ensure the light is positioned appropriately to avoid overheating the water. Approximately 6-8 hours of light per day is generally sufficient.
How do I cycle a new 21 litre aquarium?+
Cycling involves establishing beneficial bacteria that convert harmful ammonia and nitrite into less toxic nitrate. This process takes several weeks; you can use a fishless cycling method with ammonia or a bottled starter product to speed it up – check Amazon UK for options.
What’s the best way to add fish to my new 21 litre aquarium?+
Acclimate your new fish slowly by floating their bag in the aquarium for about 30 minutes to equalize temperature. Then, gently release a small amount of water from the bag into the tank and allow them to adjust gradually over several hours before adding the rest.
What are the common problems I might encounter with a 21 litre aquarium?+
Common issues include algae growth, ammonia spikes (indicating poor cycling), and temperature fluctuations. Regular maintenance, proper filtration, and careful monitoring of water parameters will help prevent these problems – consult Amazon UK for relevant equipment.