No harm in leaving the tank completely empty, it is partially full that you should be worried about.
A little gunk in the tank is normal. If it is green and slimy, that can be algae which is not good. Biocide will deal with this.
A good way to tell the condition of your tank is to cut open the fuel filter after you change it each time. By looking at what is in it, you can get an idea of the condition of the tank. You will always find something since diesel tends to have all sorts of junk in it.
A little gunk in the tank is normal. If it is green and slimy, that can be algae which is not good. Biocide will deal with this.
A good way to tell the condition of your tank is to cut open the fuel filter after you change it each time. By looking at what is in it, you can get an idea of the condition of the tank. You will always find something since diesel tends to have all sorts of junk in it.