Its a never ending battle im afraid -- I got a 125gal and I have two Aquaclear 500s filtering it and I still do a 30-40% water change on it every 5-6 days. Its all worth it I love having big fish>
They make vaccums that suck the stuff from the bottom without changing the water. If they dont sell them in your local pet store, try a catlog company like Big Als or Liveaquaria.com.