thats the first sign that the media is getting used up and needs changing, is when your constantly readjusting the bubble count and drip rate after the initial setup and break in period, also when you lose about a third of your media it's time to change the media.
when i add new media i wait a couple weeks for it to settle then put a line on the reactor when the top of the media is and when it drops a third or more thats when i change it.
you probably waited to long to change the media and thats why you think it's not effecient anymore, add new media and you should see a great improvement.