Terry is right. Unless the tanks are leaking or badly corroded, leave them in. My tanks are black iron. About three years ago, my filters started clogging up with rust and algea. I took them to have the tanks washed and they responded well to the treatment. There are those who say that tank washing is a wast of money. On some boats it is; but on this particular boat it seems to work well because of the way the tank is shaped. The tanks have a narrow bottom where all the crud collects and the suction hose can get to it. Also as they pump out each tank to clean them, the boat lists so the fill side is elevated. This allows the person cleaning the tank to pump solvent into the fill side and wash it over to the side where he has his suction hose. On my boat, I could here all the junk flowing through the hose like sand. I still have to change a filter more often than if I had changed the tanks, but the tank wash realy extended the filter life to where it is manageble and I didn't have to chop up the boat.