how to weight boat for slower speeds?

I was on vacation last week and pulled up a couple of young kids for their first runs on a wakeboard ( 7 and 9 ). I have an 02 X-star. Had the ballast empty and was going 18mph. Wake was really washy. Any way to clean a wake up at slower speeds? Or should I just run them closer to 20 where the wake cleans up? I just figured I'd go a little slower seeing it was their first time. That washy wake at slower speeds seems to eat up there board when they try to cross the wake!