... water preserves moisture in the tissue causing it to decompose SLOWER.

it is all about the way the coroner said what he said...

easiest answer would have been:
she drowned in there (..with most possible option: got in the tank by herself)
but she did not have signs of drowning, conscious or unconscious, so this was ruled out.

next best answer would have been:
she died of lack of oxygen (still with the option, she got in the tank herself)
he did not say that - because they ruled this out as well.
no signs in/on her body for that (eg if she was trapped, she would have marks from scratching the inner tank on her fingers, broken nails, rust under the nails and stuff)

what´s left after that:
she died, but with no signs from dying in the tank due to drowning (conscious or unconscious) or lack of oxygen.
so dying in the tank can be ruled out.

what´s left now:
she died. outside the tank. reason yet unknown. therefore tests of toxics (and other stuff..)
but died not long ago before she was put in the tank, as she would have been much more decomposed if her body would have been dead outside of water for longer time (like days, a week). being dead in water somehow conserves the body.