Built by hobbyists, for hobbyists

There is a moment every planted tank keeper knows: you have just switched your CO₂ on, your lights are off, and for the next two hours the tank is quietly doing its thing. The needle valve is set, the diffuser is spinning out a steady chain of microbubbles, and somewhere in the back of your mind you are doing mental arithmetic — is my pH dropping fast enough? Am I going to hit 30 ppm before lights-on?

That mental arithmetic is what AquaCalc exists to replace. Not because the maths is difficult, but because you should be watching the tank, not staring at a spreadsheet.

A hobby that rewards obsession

Planted aquariums sit at an unusual intersection. On one side is pure aesthetics — the art of aquascaping, the sweep of a Dutch street, the negative space of an iwagumi, the chaos of a blackwater biotope. On the other side is hard science: carbonate chemistry, CO₂ solubility curves, nitrogen cycling, the relationship between light intensity, CO₂ concentration and plant growth rate.

Most hobbyists come for the aesthetics and stay because the science turns out to be genuinely fascinating. Understanding why your KH matters, or what is actually happening in your filter media, or why aquasoil tanks behave differently from hard tap water tanks — these are rabbit holes that reward curiosity. AquaCalc grew out of that curiosity.

The first version was a single-purpose CO₂ calculator: enter your KH and pH, get a ppm reading. Useful, but limited. Over time, as the tanks in front of us grew more complex and the questions got harder — how much substrate do I actually need? what does my water report mean in practice? is my filter handling the fish load? — more tools got added. My Tank and the wider toolkit that exist today are the result of that process: 14 free calculators, plus the My Tank dashboard — all free.

Great tools should be free

This is the one principle that has never changed. Planted aquariums are already an expensive hobby — quality substrate, pressurised CO₂, good lighting, decent test kits. The last thing anyone needs is a paywall on a pH calculator.

For personal use, AquaCalc needs no payment and no account. There are no ads. Everything works anonymously the moment you arrive — your tank parameters, your pH readings and your water chemistry profiles live in your browser and on our server under a randomly generated anonymous ID, with no name or email attached. Used that way, we genuinely cannot link your data to you. That is by design.

The one thing that has been added is choice. If you would like your tanks to follow you from your phone to your desktop, you can optionally create a free My Tank account. It is passwordless — you sign in with a one-time link or code, so there is no password to manage — and the only things ever stored against it are your tank data and the email you sign in with. Nothing more, and only if you choose to.

AquaCalc for personal use is, and will remain, free. Separately, we offer AquaCalc for Business — a service for aquarium shops that want to embed our tools on their own websites, with a free Community plan plus optional paid upgrades. That is a distinct product for retailers; it changes nothing about the free tools you use here.

We believe the planted aquarium community is better served by open, accessible tools than by gate-kept ones. If a new hobbyist setting up their first CO₂ system can find their target pH on a free tool and dial in their needle valve correctly, that is one fewer fish killed by CO₂ overdose. That matters.

We tell you what we don’t know

The hobby is full of confident advice, and a surprising amount of it is folklore — repeated so often it has come to feel like fact. We work hard not to add to that pile. Every guide we publish is built on real sources, and each carries a small badge saying how it is grounded: Peer-reviewed sources when its claims rest on published research we cite in full, or Cited when it leans on established references and hobby consensus. (The badge describes the sources — we would never claim the article itself was peer-reviewed.)

More important than the badge is the line we draw down the middle of every topic. Where the science is settled, we lean on it and show you the papers. Where the hobby “knows” something the evidence does not actually support, we say so plainly — this is widely repeated but never tested, this is a reasonable inference, not a proven result, this is folklore. We would genuinely rather tell you “nobody has measured this” than dress a hunch up as certainty.

The same care goes into Ask AquaCalc, the answer box at the top of every page: it replies from our own guides and profiles and shows you where each answer came from, rather than inventing a confident-sounding paragraph from thin air. If we cannot back something up, we would rather say so.

The hobby we love

Fish keeping as a whole is one of the most diverse hobbies on earth. Nano shrimp tanks. African cichlid biotopes. Blackwater rivers replicated in living rooms. Dutch aquascapes that take years to mature. Brackish estuaries. Goldfish ponds. Every one of these has its own community, its own body of knowledge, its own arguments about the right way to do things.

Planted aquariums, and specifically high-tech planted tanks with CO₂ injection, are a particular corner of that world that we find endlessly interesting. The combination of plant biology, water chemistry, aquascaping aesthetics and livestock care means there is always something to learn, something to tweak, something to improve. No two sessions are the same. No two tanks are the same.

A worldwide community

AquaCalc users come from across the globe — from dense urban aquascaping scenes in Europe and North America to growing planted tank communities in South-East Asia and beyond. The map below shows the top 10 countries by visitors over the past 12 months, based on aggregated, anonymous analytics from Plausible — no personal data is used.

And wherever they visit from, they gravitate to the same few corners. Right now the guides draw the most attention — Paludariums is the most-read, with the Pearlweed profile a close second in Plantbase, and of the tools, the aquarium heat forecast gets the heaviest use.

We are all still learning

One of the things we love most about this hobby is that expertise is accessible. The person who has been keeping planted tanks for twenty years is usually happy to spend an hour helping a beginner understand why their plants are melting. The forum threads that shaped how we think about CO₂ monitoring were written by people who had no obligation to share what they knew — they just did, because that is what the community does.

AquaCalc is built in that spirit. The tools here exist because other people shared their knowledge freely — through forum posts, through open research, through decades of collective experimentation. We are returning that knowledge in a different form: practical, immediately useful, and permanently free.

If something is wrong, incomplete or missing, we want to know. The feedback page is genuinely read. If you have a tool idea, a correction to something we have written, or just a better way of explaining something — tell us. The best version of AquaCalc is the one that reflects the community's actual needs, not our assumptions about what those needs are.

What is next

The current toolkit runs to 14 free calculators — side-by-side fertiliser comparison, tank volume, substrate planning, the Tank Kit Builder, CO₂ cylinder runtime, drop checker reference, filter capacity, a pH method finder, fish and plant species lookup, a UK shops and suppliers map, parameter logging, a maintenance schedule and aquarium heat forecasting — plus the My Tank dashboard for CO₂/pH monitoring, RO water mixing, fertiliser dosing and water changes, alongside 73 in-depth Plant Insights plant profiles and a growing library of 38 science guides. And if you would rather just ask, Ask AquaCalc — the search box at the top of any page — answers hobby questions in plain English, drawing on those same guides and profiles and citing its sources. That is a lot of ground, but the hobby is wider still.

We are always looking at what people are calculating manually — on scraps of paper, in forum posts, in their heads at 6am before CO₂ turns on — and asking whether a tool could make that easier. Not every idea is worth building. But the ones that genuinely save time, reduce error, or help a new hobbyist understand their tank chemistry better? Those are worth building.

Thanks for being here. Now go look at your tank.

Explore the free tools → Read the guides → Browse Plantbase →