warm soapy washing up liquid solution - lots of bubbles/suds
perfect degreaser for removing any machine oil residue
& maybe old toothbrush to scrub it up, rinse well in cool water should suffice
(unless there is a really heavy s/hand tank with rank juice tang in it)
some tanks are fine new out of box - others have a horrid tang to them even after a quick clean
then there are tanks that seem to have a kinda metalic taste to them - cheapo clone ce4's come to mind
usually it should subside after a few rinses, refill, rinses refill (eventually in some cases)