In reality it is impossible to get crystal clean gravel. All the mulm, food particles and waste gradually slip between the gravel particles and even a really good siphon will never get rid of all of it. And as long as you are cleaning out on a weekly basis it will never become a problem. If it really bothers you loads then change to a sand substrate. In this case the waste all sits on the surface which always tends to get messy looking quite quickly. 40% water changes a week are also rather excessive, really you should aim for 20% ish unless you have particularly messy fish or are over stocked. There are also battery powered gravel vacs which suck water up, pass it through a fine filter bag and send it back into you tank again. These are good, as you don't lose water as you are siphoning but I haven't used one myself so wouldn't want to recommend a particular one to you.