Aquarium Stocking Calculator
Work out your tank's water volume and get two independent, transparent rules of thumb for how much fish it might comfortably hold. Freshwater tanks only.
Tank dimensions
Expected adult fish size
Fish body type
Two independent estimates, shown separately — they can genuinely disagree for tanks of the same volume but a different shape.
Before you stock
- Cycle the tank before adding fish — growing the beneficial bacteria that convert toxic ammonia into safer nitrate typically takes several weeks (or under a week with bottled bacteria and an ammonia source). Adding fish to an uncycled tank is one of the most common causes of early fish loss.
- Add fish gradually, not all at once, even in a fully cycled tank — a sudden bioload spike can outpace your filter's capacity.
- Plan around each fish's adult size, not the size it is at the store — a "small" fish today may need considerably more room as an adult.
- A weekly water change of roughly 20-30% is a solid default for a typical community tank; lightly stocked or heavily planted tanks can often get away with less, more heavily stocked tanks may need more.
- Filtration and maintenance matter more than any single stocking number — a well-filtered, diligently maintained tank can comfortably support more than these estimates suggest, and a poorly filtered one can be overstocked well below them.
- These estimates are for freshwater tanks only. Saltwater and reef tanks follow meaningfully more conservative stocking norms due to higher water-quality sensitivity and territoriality — don't apply this calculator's numbers to a marine setup.
How this is calculated
Water volume comes straight from your tank's dimensions, minus an adjustable allowance for substrate and decor (a rough, commonly-used estimate, not a precise constant). From there, two independent, widely-known rules of thumb estimate how much total fish length that water might comfortably support. The volume rule is a metric-converted, size-tiered version of the classic "inches of fish per gallon" guideline — worth knowing, but treated with real caution here, since a fish's actual mass and waste output scale up much faster than its length as it grows. The surface-area rule instead looks at how much open water surface is available for gas exchange, using your tank's footprint (length × width) independent of its height. The two numbers can disagree — a tall narrow tank and a short wide tank holding the same volume of water have identical volume-rule results but different surface-rule results, which is exactly the kind of thing a single blended number would hide.
Common questions
Why does this calculator show two different numbers instead of one final answer?
Because they measure different things and can genuinely disagree. The volume rule is a simplified, size-tiered take on the well-known "inches per gallon" guideline; the surface-area rule instead reflects how much open water is available for oxygen exchange, which depends on tank shape, not just volume. Blending them into a single number would hide that disagreement — and neither rule accounts for filtration, species behavior, or bioload, which matter more than either number alone.
Does this work for saltwater or reef tanks?
No — this calculator is scoped to freshwater tanks only. Saltwater and reef tanks are meaningfully more sensitive to water quality and are typically stocked far more conservatively (roughly one small fish per 5 gallons is a commonly cited reef guideline, versus roughly one inch per gallon for freshwater). Applying these freshwater numbers to a marine tank would significantly overstate what it can safely hold.
Why doesn't the surface-area rule change when I adjust the tank height?
By design — that rule is about how much water surface is exposed to air for gas exchange, which depends only on the tank's footprint (length × width), not how deep the water is. It's a deliberate contrast with the volume rule, which does scale with height — seeing both side by side shows how tank shape, not just size, affects stocking.
Part of Hobby & Craft Calculators