What's your current temperature, pH, ammonia, nitrite, nitrate?
I'd say just turkey baste your rock and put a sponge filter on your powerheads to pick up big floating debris.
You want the stuff that's going to die to die, but you don't want it to kill the stuff that's going to survive... (Yes, I know that this sounds awfully vague, but that's the theory...)
When you do a water change, be sure that your new water is equilbrated to the same temperature as the old. Many folks don't bother matching temperatures, but I believe that it is important to do so when you're changing 10% (or more) of the water.
If you feel you are having high ammonia levels, don't pussyfoot with a 10% water change. Do one as large (e.g. 25 to 50%) as you can with your equipment.
What's a 300 cascade filter? Also, I'd personally add 1 more powerhead (to have 3 running in a 55g, not 2).