Aquarium stocking and filtration calculator
The old inch-per-gallon rule is crude but it is at least a rule. Surface area and filtration matter more than volume.
L/h
%
Stocking level
1200.0%
- Filter turnover Four to six times an hour suits most freshwater tanks.
- 5.00 times per hour
- Fish length per litre
- 0.6000 cm/L
- Fish length per square decimetre of surface
- 2.222
- Weekly water change
- 50.0
- Water used per month
- 217
- Stocking headroom
- -110.0
- Surface area
- 5400 cm2
Show the working
- Water volume = 200
- Tank length = 120
- Tank width = 45
- Number of fish = 20
- Average adult length = 6
- Filter flow rate = 1000
- Weekly water change = 25
- total_fish_cm = 20 * 6 = 120
- Surface area = 120 * 45 = 5400
- Fish length per litre = 200 > 0 ? 120 / 200 : 0 = 0.6
- Fish length per square decimetre of surface = 5400 > 0 ? 120 / (5400/100) : 0 = 2.222 22
- Stocking level = 0.6 / 0.05 * 100 = 1200
- Filter turnover = 200 > 0 ? 1000 / 200 : 0 = 5
- Weekly water change = 200 * 25/100 = 50
- Water used per month = 50 * 4.348214285714286 = 217.411
- max_fish_cm = 200 * 0.05 = 10
- Stocking headroom = 10 - 120 = -110
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- a common guideline is 1 cm of adult fish per 20 litres, which is 0.05 cm per litre
- turnover = filter flow / tank volume
Length rules ignore body mass. A 10 cm goldfish produces far more waste than five 2 cm tetras.
Rated filter flow is measured with an empty filter and no head. Real flow is often half of it.