It's just odd that I haven't even pulled it up in a year and deep underwater it somehow cracked.

I pull mine in with the diffuser hose. Not the best idea but it works.

My first thought was I had over tightened the diffuser or something, but the crack doesn't go all the way up the threads.

I just cut off the one arm and capped it for the time being. I was going to take off one of the diffusers for my hypolimnetic aeration project anyways and I am only pushing a little over 1 CFM through the base and remaining diffuser.