This is a discussion on too much co2? within the Beginner Planted Aquarium forums, part of the Beginner Freshwater Aquarium category; -->
i just setup my diy co2 kit onto a power head on my 30 gallon tank, it would make bubbles 1 every second, sometimes ...
Some common reasons for greenwater is too much light, direct sunlight, or a water parameter problem. CO2 does not cause greenwater. If could be that with the CO2 being added, other ferts are being taken up faster, causing one to drop out. Also, Excel should be dosed daily, not once a week. The way it works, if dosed once a week, it's not doing you any good at all.
If you have green water, you may not clear it by water changes alone - water changing will just keep feeding the algae bloom. You may have to resort to using a UV filter or doing a blackout for a few days in order to clear this.
However, as indicated, the causes will remain if you don't get to the bottom of the outbreak. In a tank where there is a lot of light, either artificial or direct sunlight, you need to maintain a balance between lights, ferts and CO2 (and plants of course) to ensure that your plants will outcompete the algae every time.