Vincent, I got the guys who were installing my solar heater to clean the water tank. This is how they did it. They first, got a clean MOP, then they scrubbed the tank with it, there is a release valve by the water tank that will release the water in the tank to one of the bathrooms. In my case it was they common bathroom that has a pipe that was just above the bath floor in the shower area. Try looking for it. It actually stands out because when you notice it's blue in color in my case and you will be wondering what it's for.
In the roof area where the water tank sits, there will be a valve that is attached to a pipe, in my case it was a blue pipe, that runs towards that particular bathroom that the other end of it will appear in the shower area.
So once the guys have scrubbed the tank with the MOP, they release the water through the outlet valve.( DON'T forget to shut off the incoming valve before this. You"ll get an idea of which valve does what when you climb into the roof space. Yes I have climbed into my roof space several times out of curiosity and also to know where the shut off valves are.)
Once the water tank is empty, shutoff the outlet valve and refill the water tank again but either 1/4 or 1/2, depends on you, use the MOP again, to scrubbed not only the bottom but generally the whole inside of the tank. Just to make sure all the sediments are scrubbed free. Took them about 1 hour or so to get the tank clean.... And I mean clean because when they ran the water out after the fourth time, it was clear water that was being flushed out. Then make sure that the water tank cover is put back on properly because after a while, approx 4 mths. later when and if you go up again into the roof space you will see a film of dust and sort of like sand material covering the water tank cover????
Not sure what your roof space is like, but if you are able to move around while up there then use the MOP to scrub the tank then flush it out. Alternative, you could just flush out the sediments, several times, but it might not be a thorough way of cleaning the tank, once and for a while!!!
The cost for them to clean the tank was RM150.
Hope this helps.