The exact same case happened to me with my Kilo project, to make the ballast tank, if that's what you want to know, you have to include some sort of a pump, I used a fan style pump in mine, the fan which circulates water, is operated by a motor through a gear link and you'll also need a cutoff valve so that when you're not operating the pump, water doesn't go into the ballast tank. Some very good ballast tanks can be bought from OTW designs, a company located in the UK,
www.otwdesigns.com
remember: the motor has to be able to go both ways! In one direction and the other!