Thanks for the thoughts, everyone. Just a few clarifications:
1) Yes, the rays were brought in for necropsy after a week in the freezer. As many people have noted, it would be better if they hadn't been frozen. David mentioned formalin and one of the vets I talked to said that the ideal thing would be to refrigerate and get them to the lab ASAP. Unfortunately, this all happened right in the middle of the Christmas holiday when labs were closed and vets weren't responding to email. I also didn't have any formalin around.
Since the rays died, I've been in touch with a veritable who's who of Wisconsin state aquatic vets. I'm kind of kicking myself for not having even thought of these people's existence until things got bad.
I think there is a good lesson here for those of us who are keeping thousands of dollars worth of these fish: have a plan for what to do if one or more dies unexpectedly. Know what resources and experts are available to you and immediately refrigerate the specimens and get them into the lab ASAP.
2) There was some concern about bacteria taking root after the rays had died (i.e. when they were sitting dead in the tank). I'm pleased to report that this shouldn't be much of a problem. I was so obsessed with these rays while they were dying that both were removed and frozen less than three minutes after their last breath.
3) I'm still not sure what this tells me. Again, it's a preliminary culture, not a final report, so hopefully more details will emerge. But I also need to figure out if these findings merely suggest the presence of an endemic bacteria or a bona fide bacterial infection. (And if it's a bacterial infection was it the primary cause of death?)
4) When the first ray died (and before any others really started looking bad) I naively thought that it had swallowed a barb shed off by another ray. In order to confirm this I performed a "necropsy" with a Swiss Army Knife. The findings were as follows: stingrays are full of a substance known as "guts." They are pretty gross. And that is about all I learned. It was nasty and potentially dangerous and I wouldn't advise that anyone do that.
5) The other ray still seems ok. Knock on wood.
6) My best guess at this point is that we're going to find out that they succumbed to an endemic bacteria that caused problems due to other stresses. I'm willing to concede that I probably packed too many fish into this tank and that I need to be more modest when I restock and also get rid of most of the non-ray stock.
