I have 2 12 gallon tanks on my Jag that are selectable, I need to replace one of the tank senders and need to drain the tank down. I figured this would be a good time to completly drain the tanks and see if there is any (or how much) water is in them. What's the best way of doing this. Also while I'm working on replacing the sender unit should I drop dry ice in the tanks for extra safety?

Thanks
Mark