How to choose the right aquarium size

Choosing a tank size is the most important decision you'll make when you set up an aquarium. It decides which fish you can keep, how stable the water will be and how much space and support you need at home. This guide walks you through it step by step.

1. Start with the fish, not the tank

Decide what you want to keep first, then choose a tank that suits those fish as adults. Many fish sold small grow much bigger, and active swimmers need length to move. Ask about the adult size and needs of every fish before you buy it, and plan the tank around the biggest and most active one.

2. More water is more forgiving

It sounds backwards, but a very small tank is usually harder to keep than a medium one. In a small volume, temperature and water quality change quickly, so a missed water change or a dead snail has a bigger effect. For a first community tank, a 2ft tank is a sensible starting point. Nano tanks (1ft and 18") are great for shrimp, a single small fish or a planted nano, but they need regular, careful maintenance.

3. Check your space and your floor

Water weighs about 1kg per litre, before you add gravel, rock and the tank itself. A 2ft tank of about 57 litres gross holds over 50kg of water, and a 4ft tank of about 247 litres weighs well over 200kg filled. Before you buy:

• Use a strong, level stand or cabinet made for aquariums. Most household furniture is not built for that weight.
• Leave room behind and above the tank for a filter, heater, lighting and your hands during maintenance.
• Keep it out of direct sunlight to limit algae and temperature swings.
• Make sure there is a plug point nearby, and keep cables below the tank level with a drip loop.

4. Footprint matters as much as volume

Two tanks with similar volume can suit different fish. A longer, wider tank gives more swimming space and more floor for plants and décor. A taller, narrow tank fits a shallow shelf but gives less swimming room. If you are choosing between an 18" Narrow and an 18" Wide, the wide tank gives you more floor space for the same length.

Black Diamond sizes at a glance

Our Black Diamond aquariums are rimless, polished-edge glass tanks made in South Africa. Sizes are length x width x height, and volumes are approximate gross volumes.

• 1ft (305 x 225 x 225mm, about 15 L): shrimp, a single small fish or a nano planted tank.
• 18" Narrow (460 x 225 x 305mm, about 32 L): nano fish where shelf depth is limited.
• 18" Wide (460 x 305 x 305mm, about 43 L): a nano community or aquascape with more floor space.
• 2ft (610 x 305 x 305mm, about 57 L): a good first community tank.
• 3ft (915 x 325 x 370mm, about 110 L): a larger community or planted tank with more stable water.
• 4ft (1220 x 450 x 450mm, about 247 L): a large community or bigger fish.

Most sizes also come in our Black Silicone Series. See the full range in Aquariums.

5. Plan for the setup, not only the glass

Budget for a filter, heater (for tropical fish), lighting, substrate, a water conditioner and a test kit. A new tank also needs time to cycle, so the filter builds up the bacteria that break down fish waste. This usually takes a few weeks. Add fish slowly, a few at a time, rather than all on day one.

Need a size we don't list?

If your space needs an unusual length, depth or height, we build custom tanks. Tell us about your project, or visit one of our three Gauteng stores to see tanks in person and get advice.