Sounds like you'll need to lower the water and cut it down; it'll put a lot of cuttings in the tank that may or may float but a media bag or a sock should catch it all in no time. My procedure would be: tape it (makes an easily visible line), scribe it (use a straight edge and a brand new blade in a utility knife, take your time maybe doing it a couple or more times so that it is easy to follow), take your time! and cut, take your time! I prefer a super sharp pull saw for surgery type work to power tools, but you say it's c2c so getting a clean start will be important.