Why not fill it with lime and dive it without touching your bailout?
If you aren't comfortable doing that then do what most homebuilders start off doing, go in on a pure O2 loop. If you flush every now and again then it is hard to go wrong.
Caveat I'm sure there are a lot of people in the world who wouldn't find it that hard to go wrong...
I've been waiting 3yrs to do Mod 1...