Stocking Calculator
The old rule of thumb — one inch of fish per gallon — treats every fish as interchangeable regardless of body shape, waste output, or behavior, and it fails badly in exactly the cases where getting it wrong matters most (see The Inch-Per-Gallon Rule Is a Myth for the full breakdown of why). This calculator works from bioload instead: it weighs tank size and filtration capacity against each species' adult size, typical waste output, diet, and temperament, rather than summing up lengths.
How to use it: enter your tank's actual dimensions or volume and your filtration setup, then add the species and quantities you're planning or already keeping. The output flags where your planned stocking is comfortably within range, where it's pushing the upper edge, and where a specific species is likely to overwhelm your filtration or space regardless of what the rest of the tank looks like.
Use this before buying, not after — a stocking plan is far easier to adjust on paper than once fish are already competing for space in an undersized tank.
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Bioload, Not Length, Drives Stocking Limits
A tank's real stocking ceiling is set by how much waste its filtration and water-change routine can process without ammonia or nitrite accumulating, not by how many inches of fish fit visually in the water. A single common pleco or a small group of goldfish can produce more bioload than a much larger number of small, slim-bodied schooling fish, because bioload tracks with food intake, digestive efficiency, and body mass — not length alone. This is the core reason the inch-per-gallon rule fails: it has no mechanism to capture any of that.
What the Calculator Actually Weighs
Five factors feed into the output: tank volume, filtration capacity relative to that volume, each species' adult size (not its size at purchase — a two-inch juvenile oscar is a very different long-term bioload commitment than its adult foot-plus length), typical diet and waste output for that species, and temperament factors that affect how densely a species can be stocked without triggering territorial stress. A heavily-planted, well-filtered tank can often support modestly higher stocking than an identical bare tank with minimal filtration, and the calculator's output shifts accordingly when you adjust filtration inputs.
Why the Output Is a Guide, Not a Hard Ceiling
Two identically sized, identically filtered tanks can have meaningfully different real-world stocking capacity depending on water change frequency, plant load, feeding habits, and how attentively a keeper monitors water quality. The calculator's output is a research-informed estimate built from typical species data, not a guarantee — treat a result near the upper edge of the range as a signal to either scale back or commit to more frequent monitoring and maintenance, not as a green light to stock right up to the number and stop paying attention.
A Practical Reading of the Output
If the calculator flags a specific species as the limiting factor in an otherwise fine stocking plan, that's usually more useful information than an aggregate pass/fail — it tells you exactly which addition is pushing things, so you can drop or swap that one species rather than scaling back the whole plan. For deeper background on why length-based stocking rules mislead in the first place, see the inch-per-gallon breakdown.
Frequently Asked Questions
Why does the calculator ask about my filter, not just tank size?
Because filtration capacity, not raw water volume, sets the practical ceiling on how much bioload a tank can safely process. Two identically sized tanks with different filtration can have meaningfully different real stocking limits.
Should I enter my fish's current size or adult size?
Adult size. Stocking based on a juvenile's current size is one of the most common ways tanks end up seriously overstocked once fish that were bought small — oscars and common plecos are frequent examples — reach their actual full-grown size.
What does it mean if a result is 'near the limit' but not flagged red?
It means the plan fits within a reasonable estimated range but has little margin for error — irregular maintenance, a filter that underperforms its rating, or slightly larger-than-typical adult growth could push it over. Treat it as workable with attentive care, not as fully comfortable headroom.
Can I use this for a planted tank differently than a bare tank?
Yes — heavier planting can modestly improve a tank's effective capacity to handle bioload through nutrient uptake, and adjusting the plant-load input reflects that in the output, though it's a secondary factor next to filtration and water changes.
Is the inch-per-gallon rule ever a reasonable shortcut?
As a very rough first filter for small, slim-bodied community fish it's not wildly off, but it breaks down badly for deep-bodied, high-waste, or large-adult-size species, which is exactly where a bioload-based estimate matters most. See the linked breakdown for specifics.