removing orig water tank
to RichH''s instructions I would add two notes:
1 Concentrated lye reacts quite vigorously with aluminum, dissolving the aluminum and liberating quite a bit of heat in the process. I believe Drano contains crystals of lye and little pieces of aluminum -- it pretty much boils when the two react.
2 If you leave a tank full to the brim of lye solution, be prepared for the possibility that the solution will expand and run out the vent line. Before leaving the setup, reroute the vent line to a place where an overflow will do no harm (say f''rinstance into a bucket containing a quart or two of vinegar)