balancing the sub is just like balancing objects on a scale itself, the ballast tank should be located in the center of floatation, to figure that out, put everything in the sub in their desired place (electronics, servos, receiver, battery, etc) drop the sub in the water, (make sure that your WTC is waterproof when you do this
) use a stick or pencil to push the sub underwater to trim level and you'll probaly notice that one end of the sub will be heavier than the other.
Move the stick along the sub until you get to one place where the sub is properly balanced from both ends, mark it and this should be where the ballast tank should be. I got this trick from the Subconcepts page, quite useful! You can go on their website to check out a few tricks too. Just go to the Sub Guide at the top.