Fish Tank Size & Stocking Calculator
Calculate aquarium volume, how many fish your tank can support, and filtration requirements. Avoid overstocking which is the most common cause of fish illness.
Aquarium Guide
Tank Volume and Stocking
Tank volume (litres) = length × width × height (cm) / 1000. Subtract approximately 10-15% for substrate, decorations, and air space. The classic 'one inch per gallon' rule: 1 inch of fish body length per US gallon of water. This is a very rough starting point — it does not account for fish bioload differences or swimming space. Better approach: consider the fish's adult size (not the shop size), waste production (goldfish produce 3-4× more waste than equal-sized tropical fish), swimming level (s
The Nitrogen Cycle
Every aquarium must establish the nitrogen cycle before adding fish. Fish produce ammonia (toxic). Beneficial bacteria (Nitrosomonas) convert ammonia to nitrite (also toxic). Nitrobacter bacteria convert nitrite to nitrate (less toxic, removed by water changes). Cycling time: 4-6 weeks from scratch (fishless cycling with ammonia source). Signs of established cycle: ammonia and nitrite read 0ppm. Nitrate reading of 5-40ppm. Common mistake: adding fish to an uncycled tank causes 'new tank syndrome
Filtration Requirements
Filter turnover rate: the volume of water filtered per hour. For tropical community fish: minimum 5-10× tank volume per hour. Goldfish and messy fish: 10-15× turnover. The filter media (biological, mechanical, and chemical) is more important than the pump speed — biological media must colonise with nitrifying bacteria. Canister filters: highest capacity, most versatile, quiet. Best for tanks over 100L. Hang-on-back (HOB): good combination of mechanical and biological filtration. Easy maintenance
Water Parameters
Most tropical community fish require: temperature 24-28°C (heater essential in UK). pH 6.5-7.5 (most community fish are adaptable). Hardness (GH): 5-15 dGH (soft to medium hard). Ammonia: 0 ppm. Nitrite: 0 ppm. Nitrate: ideally under 40 ppm (water changes manage this). UK tap water: typically pH 7.0-8.0 and hard (GH 10-20 dGH). Suitable for many fish but may need buffering for soft water species (discus, some tetras). Test kit: API Master Test Kit is the gold standard for home aquariums — liquid
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