The chlorine in normal untreated tap water will likely react with any ammonia left but if you wish to be a bit more sure, add a 1/2 cup or so of bleach and it will kill any nasties and then be gone after a rinse and dry. Since we can never be certain what people may have had in used tanks, I like to start off knowing that they are safe. Like washing my plate before eating, there may not be anything there but I feel better KNOWING they are clean when I do it myself.
I once bought a tank that had strange circles in the bottom. A bit of checking and I found it was from oil cans being kept in the empty tank!
While many still fear using bleach, it has the wonderful ability to gas off and blow away, making it really easy to clear once it has done the work.