Fuel pump retaining ring?

Im replacing my fuel pump, and I can't figure out how to take off the big black retaining ring. Do you just twist, or push down and twist? or do something else all together... its a 00 jetta,I used all my strength. am I just missing something? thanks for your help :iceslolan

About the 00 Jettas I dont know. I have a 90 Jetta, and the ring just unscrews. Getting it to start turning is not easy however. I used a blunt screwdriver and a hammer, to whack sideways (as much as I could) on the raised ribs of the ring. I went around in a circle, tapping it in a counterclockwise direction, and it eventually loosened up the grip of the plastic ring on the plastic tank threads. And it was just as much trouble to get it started back on the threads without being cross-threaded. Coating the threads with windex helped a lot.
They make a 'spanner' tool to do that, but Im way to cheap to spend the money.