gravel cleaning or changing

hi

my tank has been set up for 5 months now and although i do a water
change every 7 to 14 days, by using a thing to suck up the dirt around
the gravel. i am beginning the think the gravel could do with better
cleaning. what is the procedure for cleaning gravel, how often should
it be done? what am i doing wrong by having such filthy gravel after 5
months - is this normal?

many thanks

f