Our Approach to Stocking
Why we built a better stocking calculator — and the science behind how it works.
The Problem with Traditional Rules
For decades, fishkeepers have relied on simple heuristics like the "one inch of fish per gallon" rule to decide how many fish a tank can hold. While easy to remember, these rules are dangerously oversimplified. They treat all fish as identical — a slim, low-waste Neon Tetra is counted the same as a stocky, high-waste Oscar, despite producing a fraction of the bioload.
These rules also ignore critical factors like body mass, metabolic rate, waste output, activity level, and territorial behaviour. A 10-inch Plecostomus produces vastly more ammonia than a 10-inch Ropefish, yet traditional rules treat them identically. The result? Tanks that are technically "within limits" but suffering from ammonia spikes, chronic stress, and fish mortality.
What these rules miss
- •Body mass vs. length — a fish's volume (and therefore waste production) scales cubically with length, not linearly
- •Metabolic differences — active swimmers like Danios have higher metabolic rates than sedentary bottom-dwellers
- •Waste production coefficients — carnivorous fish produce more nitrogenous waste per gram of body weight than herbivores
- •Territory and space requirements — some species need swimming space far beyond what their bioload would suggest
How Our Calculator Works
Our default calculation method — which we call the Fishi Bioload Model — takes a fundamentally different approach. Rather than reducing a complex biological system to a single linear measurement, we model the actual bioload each species places on the aquarium's nitrogen cycle.
Species-Specific Bioload Values
Every species in our database is assigned an individual bioload coefficient that accounts for its adult size, body shape, metabolic rate, diet type, and typical waste production. These values are researched and calibrated per species — not derived from a generic formula.
Waste Production Modelling
Ammonia output varies dramatically between species. A carnivorous Cichlid produces significantly more nitrogenous waste than a similarly-sized herbivorous Pleco. Our model weights each species' dietary profile and digestion efficiency to estimate real-world waste load.
Activity & Metabolism
High-energy species like Tiger Barbs and Zebra Danios consume more oxygen and produce more CO₂ and waste than sedentary species. Our coefficients incorporate activity levels so that a tank full of active swimmers is stocked more conservatively than one with calm, slow-moving fish.
Minimum Space Requirements
Beyond bioload, every species has a minimum tank size based on its swimming behaviour, territory needs, and social requirements. Our calculator enforces these minimums and warns you when a species simply won't thrive in your tank, regardless of remaining bioload capacity.
Filtration Adjustments
The calculator also supports filtration level adjustments. Heavy or over-rated filtration can process more waste, effectively reducing the bioload impact on water quality. Our model applies a conservative multiplier to account for enhanced mechanical and biological filtration — but it never overrides minimum space requirements, because filtration can't solve the problem of a fish that simply needs more room to swim.
Our Species Database
Our calculator is backed by a database of over 900 freshwater species, each with individually researched parameters. This isn't scraped or auto-generated data — every species profile has been reviewed against established ichthyological references, aquarium literature, and community-validated husbandry knowledge.
For each species, we track:
This data is the same database that powers the Fishi mobile app, used by over 100,000 fishkeepers to manage their aquariums. It's continuously updated as new species become available in the hobby and as our understanding of existing species improves.
Three Methods, One Calculator
We believe in transparency. Our calculator offers three stocking methods so you can compare approaches and make informed decisions:
| Method | How It Works | Accuracy |
|---|---|---|
| Fishi Bioload Model | Species-specific bioload coefficients based on waste production, metabolism, body mass, and activity | Recommended |
| 1 Inch per Gallon | Traditional rule — 1 inch of fish length per US gallon of water | Rough Estimate |
| 1 cm per Litre | Metric equivalent — 1 cm of fish length per litre of water | Rough Estimate |
The traditional methods are included for reference and comparison. We always recommend using the Fishi Bioload Model for the most accurate stocking assessment.
Limitations & Responsible Use
No calculator can perfectly model a living ecosystem. Aquariums are complex systems influenced by filtration efficiency, plant density, feeding habits, water change frequency, temperature stability, and dozens of other variables that can't be captured in a single number.
Our calculator is a planning tool — a starting point for making informed decisions. It should always be used alongside your own research into each species' specific needs, behaviour, and compatibility. Individual fish can vary significantly in temperament, and tank dynamics can shift as fish grow and territories form.
When in doubt, stock conservatively. An understocked aquarium with excellent water quality will always produce healthier, more vibrant fish than an overstocked tank pushed to its limits.
Ready to Plan Your Tank?
Put our bioload model to work — try the calculator with your species