Advanced options
Stocking guides (like the old 1cm of adult fish per litre) are rough starting points for small community species only: research each species' adult size, temperament and shoal needs. Cycle the filter for 4-6 weeks before fish, and never wash filter media in tap water.
What The Aquarium Calculator Works Out
- Tank volume: length x width x water height in centimetres, divided by 1,000, with the UK gallon equivalent (4.546 litres). A 100 x 40 x 50 cm tank holds 200 litres gross, which is also 200 kg of water before the glass, gravel and stand are counted.
- Real water volume: the gross figure less 10% for substrate, rocks and wood. This is the number that matters for dosing medication and dechlorinator and for stocking.
- Kit sizing: heater at about 1 watt per litre, filter turnover of 4 to 5 times the volume per hour (canisters for big tanks, internals for small), and a weekly 25% water change scaled to the real volume.
- Units and tank type (advanced): enter dimensions in inches if the tank was sold that way. Coldwater tanks drop the heater; marine tanks add a salt card, heavier water and a more conservative stocking guide.
- Deduction, heater, turnover and water change (advanced): the four rules of thumb the simple mode hard-codes, each editable: a heavily planted or rock-scaped tank may lose 20% to decor, a tank in a cold room may want 1.5 W/L, and heavily stocked tanks run 8 to 10 times turnover.
- Electricity per month (advanced): the heater (assumed on about 35% of the time in a heated room), the filter pump (about 1 W per 100 L/h) and lighting for 8 hours a day, priced at the current price-cap unit rate (editable). A 200 litre tropical tank typically costs GBP 5 to GBP 8 a month, more in winter than summer.
- Total weight on the stand and footprint (advanced): water (1 kg per litre, 1.025 for salt water) plus the glass (from the thickness you enter, at 2.5 kg per litre of glass) plus the substrate, and the resulting load per square metre of floor. A quarter of a tonne is why big tanks stand against walls, across joists, and never on flat-pack furniture rated for a television.
- Marine salt or water change buckets (advanced): for marine tanks, the salt needed to mix the full volume at 35 g/L and per water change; for freshwater, how many 10 litre buckets each weekly change takes and the tap water used over a year.
- Stocking guide (advanced): the old 1 cm of adult fish per litre rule (1 cm per 4 litres for marine) applied to the real volume, with examples. It only gestures at small community species: adult size, shoaling numbers and temperament decide the real answer, and many "beginner" fish need six or more of their own kind. The universal beginner error remains the goldfish, a pond-sized fish sold at bowl size.
- Fill-level table (advanced): volumes and weights at 50%, 75% and 90% fill, useful for tanks with a low waterline or when planning a part-fill for a move. Remember the invisible chemistry: a new filter takes four to six weeks to grow the bacteria that convert ammonia to nitrite to nitrate, and media washed in tap water dies with its chlorine.