low alkalinity

[Snaz]

If I were you I would start a new thread describing your lid and how it fits and your concerns regarding gas exchange/evaporation etc.

There are lots of people here with experience on lids. I think this thread has run it's course. Also nano-reef.com is the premier site regarding nano tanks. Try the hidden google search on that site to get any answer you need nano wise.
http://www.nano-reef.com/search/

Also the dead crab might have been a molt. With a flashlight look at your rocks a few hours after the lights are out and you will experience a whole new side of your tank you haven't seen before. Good luck with the reef!