Whenever I do water changes with my siphon and gravel vac attachment (it's a hard cylinder that has a larger diameter than the siphon tube), I try to vacuum the gunk out of the gravel but all I do is suck up the gravel and clog the siphon. Am I just vacuuming wrong? I put the vacuum end in the water, suck the other end and then push the vacuum into the gravel. If that's right, what can I do to fix it? The fish poo is starting to build up and I don't want to cause a disaster when I can actually vacuum for the first time.