If you are able to siphon the sand out of the tank without removing the rocks, that means you have the surface area to thoroughly clean the sand in the tank. (Without having to remove the sand)
Since your sand bed is fairly shallow, you hopefully won't be disturbing an anaerobic/dangerous layer underneath by cleaning it via siphoning. When I do a water change- and this keeps my sand-bed looking fresh - I siphon the sand bed using a hose that?s attached to a tall cup/vacuum ?the ones that come with any standard fish tank cleaning kit. How I do this is:
once you start the siphon, you will then insert the cup/vacuum into the sand, it will quickly fill up with sand and water, as the sand fills the cup you will notice detritus/smaller particulate matter rising out of the swirling sand to the top faster and getting sucked out while the denser sand is at a slower pace filling up the cup. Before the sand gets through the cup, into the hosing and out of the tank, you will need to quickly have one of your hands ready to crimp the hose - this will temporarily pause the siphon and allow the now clean sand to fall bank into the tank, once the sand is out of the cup repeat this process throughout the sand bed and VOILA your sand bed is looking brand new!
(if you were to remove the sand I?d just flat out replace it and not hand wash it ? not worth the work)
