Would the drop check method work in a SW tank? I'm inclined to think it will since we don't use tank water. I've no intention of injecting CO2 in the SW tank, but i'm just curious.

It should work the same in salt water as in fresh water. Or, at least I can't think of a reason why it wouldn't.

Transfer rate of gas might be slower, but it should work the same.

You can also modify and adapt this idea to O2 levels and test kits as well
Or any gas of interest.......

Regards,
Tom Barr