Built by fish keepers.
For fish keepers.
Tank Logic is a free, independent aquarium resource. No paywalls, no sponsored recommendations, no fluff — just accurate tools and honest guides built by people who actually keep fish.
Demystifying aquarium math, so the maths never kills a fish.
Almost every avoidable failure in this hobby comes down to numbers nobody explained properly: how much water a tank actually holds, how much bioload a filter can really process, how many fish a footprint can support. Get those numbers wrong and the consequences arrive on a biological schedule — ammonia doesn’t negotiate.
Tank Logic exists to make those numbers effortless. Every calculator is built to give you a clear, accurate, instantly-usable answer — in litres and gallons, centimetres and inches — so you can make stocking and setup decisions with the confidence of someone who’s done the maths, without doing the maths.
No paywalls. No accounts. No sponsored recommendations. Just tools that work.
Because the inch-per-gallon rule deserved to be retired.
Tank Logic is an independent, data-driven project built by a long-time aquarium hobbyist who also happens to write software — born from the gap between how fishkeeping is usually taught and how aquariums actually behave.
Most stocking advice still leans on linear rules of thumb — the infamous “one inch of fish per gallon” chief among them. It’s a rule that treats a slim, mid-water tetra and a chunky, waste-heavy pleco as interchangeable, ignores filtration entirely, and says nothing about whether a fish has the swimming terrain it needs. It survives because it’s easy, not because it’s right.
Our calculators were built to move past that trap: dynamic, multi-variable tools that weigh bioload capacity, filtration performance, and swimming terrain requirements — the factors that actually decide whether a stocking plan thrives or crashes. The same philosophy runs through everything here, from the nitrogen-cycle guides to the genetics calculator: model the biology, not the folklore.
And it’s grounded in real tanks. The fishroom behind this site has run planted Walstad setups, unheated cold-water systems, and an active bristlenose pleco breeding project for over six years. Every photo on the site is from those tanks. When a guide says something works, it’s because it worked — and where something failed, we say so.
Lightweight, fast, and free. Permanently.
Tank Logic is built as plain, fast, dependency-light web tooling — no app installs, no accounts, no trackers beyond the basics, nothing between you and the answer. Pages are designed to load quickly on any phone, anywhere in the world, on any connection.
Every calculator and guide on this site is free, and will stay free, for the global hobbyist community. If a tool here helps one more tank get cycled properly before the fish go in, it’s done its job.
Written to be useful, not just to rank.
We publish long-form guides on aquarium topics where beginner advice tends to be scattered, contradictory, or incomplete. Every guide is written with a single goal: give the reader a clear, practical understanding of the topic so they can make a good decision.
Our 25+ guides cover tank cycling, water chemistry, choosing tank sizes, fish stocking, planted tanks, the Walstad method, fish care, disease identification, and more. Word counts range from 1,000 to 2,000 words. Every guide includes a table of contents, callout boxes for important warnings or tips, and links to related tools and articles.
We update guides when best practices change or when readers point out errors. Fishkeeping science and community consensus does evolve — the one-inch-per-gallon stocking rule, for example, is widely considered outdated by experienced hobbyists, and our stocking calculator reflects that.
Tools that give you answers, not more questions.
Tank Size & Volume Calculator
Enter your tank dimensions and get the exact volume in gallons and litres instantly. Supports rectangular, bowfront, and hex tank shapes, with eight common size presets and smart follow-on recommendations for stocking, substrate, and water changes.
Fish Stocking Calculator
Raw volume is a starting point, not the whole picture. Our stocking calculator weighs bioload capacity, filtration performance, and swimming terrain — with the legacy inch-per-gallon figure shown only as a cross-check.
Walstad Soil Calculator
The Walstad method uses organic potting soil beneath a sand cap to feed plants directly from their roots — no CO₂, no liquid fertilisers. Our calculator tells you exactly how much soil and sand you need for your tank dimensions, with guidance on the Father Fish mineral layer variation and filtration recommendations.
Fish Care Database
Care sheets for 36 freshwater fish and invertebrate species, covering water parameters, temperament, diet, tank size requirements, and compatibility notes. Designed to be scanned quickly at a glance rather than read as an essay.
We take accuracy seriously.
Every calculator on Tank Logic uses established formulas or methods. The tank volume calculator uses straightforward geometric volume calculations. The stocking calculator weighs swimming terrain (the surface-area method) alongside bioload and filtration modifiers, with literature-based species sizes; the legacy inch-per-gallon figure is displayed only as a rough cross-check. The Walstad soil calculator follows the methods set out in Diana Walstad's Ecology of the Planted Aquarium and the practices popularised by the Father Fish YouTube channel.
Our guides are written by people who have actually kept aquariums and made the mistakes beginners make. We cross-reference our information against established aquarium resources, academic literature where relevant, and the broader fishkeeping community.
That said, we are not infallible. If you spot an error — in a calculation, a recommendation, or a care sheet — please tell us. We will investigate it and update the content if needed, and we credit corrections where appropriate.
Display advertising. Nothing else.
Tank Logic is supported by display advertising through Google AdSense. When you browse the site, you may see advertisements in certain areas of the page. These ads are served by Google and help cover our hosting and development costs.
We do not accept sponsored content. We do not write reviews in exchange for products. We do not receive affiliate commissions from product recommendations. Our tool results and guide recommendations are not influenced by any commercial relationship.
If you find Tank Logic useful, the most helpful thing you can do is share it with someone new to the hobby. Word of mouth is how independent resources like this survive.
Get in touch
Questions, corrections, tool suggestions, or just want to tell us about your tank? We read every message.
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