Cleaning Diesel Tanks

Thanks for all your replies everyone, i think from what you have all said i will probably have to filter the old fuel and re-use it after scrubing the tanks clean. i will probaby put some diesel addatives in the fuel as well to clean the fuel system and kill the bugs, this is mainly because of budget i dont think i can afford to spend to much more on fuel than i have to (but obviously if its going to be dangerous for me not to change the whole lot then i will have to). what do you think about diluteing some of the fuel with fresh and say only getting rid of 25-30% of the old fuel (still filter & clean the tanks of course)? do you think i would gain anything by doing this? i guess by the time ive filtered off the muck and water etc ill loose quiet abit of fuel anyway?
thanks for your help

Ed