There should be an expiration date on the bottom of the container of Kanaplex. If there isn't an expiration date, there will be a lot number, call 888-Seachem to find out if it's still good.
You can use Kanaplex with Focus as a binder, and treat the food - there should be directions for that on the label, and you can do that in the display without any ill effects.
If you plan to treat the water (not the food) then a quarantine/hospital tank would be a must.
Jenn