Some people just top off with new media. What I do is I wait till it gets down to between 1/2 and 1/3 full. By then the remaining media will look gritty and broken up. When it looks like that, I use my buckethead wet dry vac and suck it all out and refill it with brand new media. It might be overkill but I’d like to think that the new media helps avoid the clogging and bogging down of the recirculating pump and lines.