[killudead]

Drop checker.

Here's ya' go:

http://www.greenleafaquariums.com/co2-drop-checkers.html

(be careful of the ones that are cheaper and tell you to use tank water in the checker, they are not accurate).

And here's a fantastic thread about them and their use (accurately!):

http://www.barrreport.com/articles/2661-drop-checkers-co2-indicators-why-how.html

article is a great read, but i wonder which drop checker is better. the double chamber or single chamber

 

[killudead]

They are both better.

better than nothing, of course, but out of the 2. which one is preferable over the other.