While in Cozumel last week, I had 2 scuba tanks that registered 7 ppm of carbon monoxide (CO). I tested a few more until I found a tank with 2 ppm, which I used, and changed my dive plan to a very shallow dive. So please don't let your guard down or fall into a false sense of security that your tank is okay. If you've got a tester, take it and use it. If you don't have a tester, think about getting one.