If none can be detected, you should add another tab per 5 gallons of water and reseal your container. Even in a sealed container off gassing will continue, but at a greatly reduced rate. I have several bottles of the stuff and love it for several reasons. But let's face it, many people just can't afford 23 per gallon for purified water. I'd a lot rather use something that would do it for only 3 a gallon.
Boiling water is certainly another alternative.
However, RediChlor is simpler that boiling and doesn't require any scarce fuel resources to work. The drawback to RediChlor is you need to purify 5 gallons of water at a time. Don says they are starting to work on making tablets designed with just this in mind, a pill to purify one quart of water. For things like this, Ion is still a really handy way to go. The two ounce bottle is easily packed along, and Ion works quickly, only requiring a wait of 4 minutes.