i was thinking, draw an imaginary line down the middle of the tank, clean half the gravel thouroughly with the water change, clean the other half with the next weeks change so as not to remove too much beneficial algae etc at once......sound ok?
If you are doing regular water changes and your filters are working normally. Then you don't really need to be washing your gravel once it's in the aquarium. You can clean it of course with one of the commercial gravel cleaners available or you can make your own from a length of tubing and cut of the base of a clear johnson's baby shampoo bottle. Attach the tube to the neck end of the bottle and use as a syphon digging it into the substrate as you go.
Hope this was of some help
Regards
BigC