I once had a Prizm by Red Sea in my 20 gal. Its cheap but works in a pinch. I guess you get what you pay for. Sounds like you've got it figured out though...
My bet is that you will never get rid of all of them by siphoning them but might as well try. Flat worm exit is going to be your next best bet. I've used it many times and it works. Just be careful.