We did that....and all the above advice is good.
I'll bet you the water you put in is from a well......ours is. No chlorine or treatment to begin with. 2oldman - that may be why you didn't have the problem. We live in Central Oregon and the heat in the summer, combined with well water over a period of time is GAGGY! Dirty diaper pail is exactly how it smelled.
I can't remember how many gallons our tank held......we put a GALLON in and flushed it thru, then another 1/2 gallon to let it set for about 2 hours. After that, the smell was gone. :)