For a rectangular tank, volume in litres = Length (cm) × Width (cm) × Height (cm) ÷ 1000. To get usable water volume, multiply by 0.9 to account for gravel, decorations and the space left at the top. In imperial: Volume (US gallons) = Length (in) × Width (in) × Height (in) ÷ 231.

Setup costs vary widely by tank size and livestock choices. A small 60–100 litre freshwater tank typically costs £150–£400 all in. A larger 200+ litre planted or marine setup can run £600–£2,000 or more. The main cost categories are the tank and stand, filter, heater, lighting, substrate, decorations, water conditioner and livestock.

A common starting rule is 1 cm of adult fish body length per 2 litres of water, though this is a rough guide only. Territorial fish, messy eaters and fish with high oxygen demands need more space. Always research the specific needs of each species, and ensure your filter is rated for your tank volume.

Gross volume is the full internal capacity of the tank calculated from its dimensions. Net or usable volume is lower — typically 80–90% of gross — after accounting for substrate depth, rocks, decorations and the headspace left at the top. Net volume is what matters for stocking levels and filter sizing.

To convert litres to US gallons, divide by 3.785 (or multiply by 0.2642). To convert to UK/imperial gallons, divide by 4.546. For example, a 200-litre tank holds approximately 52.8 US gallons or 44.0 UK gallons.

As a general rule, your filter should turn over the full tank volume at least 4–6 times per hour. For a 100-litre tank, choose a filter rated for at least 400 litres per hour (l/h). For messy fish or heavily stocked tanks, aim for 6–10× turnover. Always check the manufacturer’s stated flow rate at actual head height rather than the maximum quoted figure.

A bow-front tank has a curved front panel that increases the front-to-back depth at the centre compared to the corners. The extra volume from the bow is approximately 15% of the equivalent rectangular volume, so this calculator uses a factor of 1.15 × (Length × Back Depth × Height ÷ 1000). This gives a reasonable estimate; the exact figure depends on the specific curve radius of your tank.

Freshwater weighs approximately 1 kg per litre at room temperature. Saltwater is slightly denser at around 1.025 kg per litre. This means a 200-litre freshwater tank, including the tank glass and equipment, can easily weigh 250–280 kg fully set up. Always ensure your stand and floor can safely bear the total weight before filling.