Round up is actually a great tank cleaner alone...it will remove all traces of 2-4D if you have any in there from your yard spraying. That said, you still have to remove the Round-up from the tank before going back. Rinse till you don't see bubbles-fill up part way, slosh around to get top and sides of tank, dump/pump out, repeat. Be sure to flush any agitation/return lines, booms, and hand wands during each step. If you want to be real sure, add some ammonia or get a tank cleaner product where-ever you buy your chemicals from and use that during one of your last flushes. I don't usually do this w/ RUP, b/c it comes out easily, but you can. Always remember to treat the flush water as chemically treated, don't do this in your yard, you'll not be happy. I tend to spray off my first tank or so of rinse on weeds around, sometimes they die, sometimes not, but it doesn't cost me much.