Color Perception - CO2 Checker
 

Sometimes our aquarium setups make it difficult to determine the color of the CO2 checker. I have come up with a idea that works real well.
Some of the office supply stores like "Staples" have a plastic heat sealing machine. This machine will sandwich something like a business card and heat seal it with stiff plastic. So I used one of their business cards and ask them to seal it. Of course, one of the sides of the business card must be blank and have a white background.
I find this works real well with no leaks getting into the card. Give it a try.