Sometimes DE is a good enough wormer. But when I see worms in thier poo, I use the wormer in their water and follow up 10 days later with the wormer paste (for horses, forget the name right now.)
I wouldn't use the wormer in the water all the time, because you have to throw away the eggs for 10 days after you use it, so all the time use would mean you never have eggs. Plus, it's really like a poison to kill the worms, and you're only supposed to use it for one day -- more than that at one time could be too hard on your birds and actually kill them.