Most people arriving at a CO2 calculator want one number, once. For that, several of the options below are a better use of your time than installing anything. Here is the comparison written the way it would be if we were not the ones selling an app.
| Option | Better at | Worse at |
|---|---|---|
| Free web calculators Rotala Butterfly, TankCalcs, AquaKit and others |
Nothing to install, works on any device, and instant. If you want one number right now, open one of these. | They generally use the flat factor of 3, which reads about 10 to 21 percent low, and they return a bare value with no error band and no validity check. |
| A printed KH and pH chart | Free, offline, and readable at a glance next to the tank. Good enough when you are only tracking direction of travel. | Fixed temperature, no tolerance, and no way for it to tell you that your soil substrate has invalidated the whole grid. |
| A drop checker alone | A physical reading rather than a computed one, unaffected by your tank chemistry, and about the price of a coffee. For many tanks this is genuinely all you need. | Broad colour bands, and it lags the tank by one to two hours, which is exactly when people misread it. |
| A pH controller | It acts rather than reports. Nothing on this page automates your solenoid. | Far more expensive, needs calibration you must keep up with, and it inherits every one of the assumptions that break on soil and peat. |
| General aquarium log apps Tank managers with a CO2 calculator among many tools |
One place for every parameter, maintenance task and reminder. If you want a full tank journal, a single purpose CO2 app is the wrong shape. | The CO2 tool is usually the same flat factor of 3 with no tolerance carried and no validity gate. |
| CO2Scape | Derives the constant for your temperature, carries your test tolerances through to a range, blocks the carbonate route when your declared substrate or additives break it, and runs the pH drop method with a timer. | It is an iPhone app, so it is one more thing to install. It measures nothing itself. The free version shows advertising. And it will refuse to answer in cases where a web calculator will happily give you a number. |
When you do not need an app
- You want a single number and you know your tank is inert, unbuffered and above 3 dKH. A web calculator will do, provided you mentally add roughly 15 percent for the temperature correction.
- You already run a drop checker and your plants and fish look right. Then you have a physical reading and a biological confirmation. That is a stronger position than any arithmetic.
- You want automation. Buy a controller. No calculator on any platform will turn your gas down for you.
When the app earns its place
- Aquasoil, peat, botanicals or a pH buffer. This is the case where a free calculator is not merely less precise but wrong by an order of magnitude, and gives you no signal that it is.
- You want the interval rather than the midpoint. Knowing the honest answer is 12 to 40 ppm rather than 22 changes what you do next.
- You are running the pH drop method properly. The degassing step needs timing, and the two failure checks catch runs that otherwise look fine.
- You keep records per tank. A shrimp tank at 2 dKH and a high tech scape at 6 dKH should never share a curve.
What it does not do
It does not measure anything, it does not replace your water tests, it is not a fish or plant database, and it does not calculate medication doses. Those are deliberate omissions rather than a roadmap.
If those cases describe your tank
Free to download, with one optional purchase that extends the record keeping rather than the chemistry. Everything on this page that concerns the calculation itself is in the free app.
