These are useful CO₂ calculators that can help you balance the CO₂/pH levels in your fish tank.
There are three tools to choose from:

Method #1

Calculate CO₂ concentration in your aquarium according to pH and KH levels.
To use this method: Simply measure the pH and KH levels in your fish tank, enter the numbers in the green frames inside the "method #1" block and you'll get your CO₂ level in ppm units.

Method #2

Calculate CO₂ concentration in your aquarium according to two mesurments of pH - before a shake and after.
To use this method: Fill a small, closable container with water from your fish tank.
Measure the pH level and enter it in the upper green frame.
Next, close the container and give it a good shake. Measure the pH level then enter it in the lower green frame.
You'll get your CO₂ level in ppm units.

pH Controler

This tool is useful if you use a pH-controller to inject CO₂.
Use this tool to figure the target pH for your controller that will get you the desired CO₂ level.
To use this method: Choose the level of CO₂ that is most suitable for the plants in your aquarium (don't forget your fish), and enter it in to upper green frame.
Next, you need to measure your KH level and apply it to the lower green frame.
You'll will get the pH level you wanted (this is what you should set your pH-controller to if you want to get the entered CO₂ level).

Note: The methods showing here could be very inaccurate - the result is very sensitive to it's input, so make sure you measured an accurate pH and KH levels. It is always recommended to repeat the measurements a few times before considering any action.