You can get name brand tank cleaners but they are targeted at bikes not cars and are not cheap. I`ve found vinegar works best, you need about 20 litres. Get a dishwashing brush and fix it to some rod so you can get into the tank and scrub it. Put 10 litres in the tank and scrub the areas you can reach, leave the vinegar in there for a week giving it a scrub and a good shake every day. After a week drain the vinegar out and if you have access to a presure washer use that to clean as much of the inside of the tank. Now put the other ten litres in and seal the tank up so it doesnt leak when turned upside down, leave it for another week (if you can get inside during the week give it a scrub).
Drain the vinegar out and then fill the tank with water , add about a quarter of a box of baking soda to neutralise the vinegar and leave for a day or so. Drain the tank and leave it in the sun to dry properly.
You should get most if not all of the muck out.