Those kind of jugs are usually used to recycle chemicals, at least that's the case where I work. Anyway, if you're sure chemicals have not been used, drop some bleach in there and fill it up with water. Let it sit for a few days and then repeat with just dechlorinator. Lastly, air them out in the sun for a few days. If still unsure, use these buckets for output purposes. In other words, don't let anything that was in it end up in your tank.
I personally prefer white open-topped buckets so I can clearly see what's in it when viewed from the top down.
Please end a sentence with a period and begin a new one with a capital letter. It will help me help you.