Generally cleaning a fish tank is very mundane and safe work, until you throw a highly aggressive fish into the mix such as the Giant Snakehead. Cleaning your tank with this monster fish in it adds a not-so-standard twist to your regular tank maintenance.
Odds are your going to be getting your Giant Snakehead as a baby, which gives you the opportunity to introduce a very useful tank mate. The Plecostomus is a bottom feeding fish that not only makes an interesting addition to your tank, but helps keep it free of algae and other waste. If you can I recommend getting a young-adult to adult Plecostomus over the baby, as your Giant Snakehead grows much faster then he will which could turn him into lunch. If introduced at an early age, or even better before you purchase your Giant Snakehead, the odds are they will stay tank mates for years to come. Try introducing a Plecostomous after your Giant Snakehead is an adult and just watch how fast you flushed thirty dollars down the toilet.
As a youth keeping the Giant Snakeheads tank clean is not an overly difficult task. I would recommend changing 10-25% of the water, based on the size of the tank, about once every 10 to 14 days. Small to medium tanks should have about 25% of their water changed, where as a much larger tank (100 gallons+) should get about 10% of the water replaced. Obviously proper filtration equipment is needed to help maintain a clean tank. On the upside, the Giant Snakehead is a very stoic fish able to survive most normal PH and Alkalinity levels so maintaining proper chemicals is not critical. The down side is as he begins to get larger and more aggressive you need to start using a great deal of caution when cleaning the tank.
First off get some very long handled brushes for scrubbing the sides clean. It is best practice to never reach your hand into the tank of an adult Giant Snakehead as they generally strike anything new that enters the water immediately. Following that I always used an electric underwater gravel vacuum from your pet store. This is a very efficient and safe method for both removing water from your tank and cleaning the gravel at the same time. It also has the added benefit of never having to reach into the tank with a bucket to remove water during the bi-weekly changes. Finally you may want to make sure he was recently fed to keep him a bit more lethargic.
Just remember to exercise extreme caution when cleaning your Giant Snakeheads tank as they are a dangerous fish. It has been said that full grown Giant Snakeheads have killed adult human beings in the wild, and though I highly doubt yours will do that you its best to be smart about tank care. The last thing you want to do is cause him unneeded stress and causing yourself some serious bodily harm.