Looks like Tank's heart murmurs are the cause of his fainting the other day. The vet thinks so too, though she says it's not normal for a dog so young to be having that problem. Tammy did some research on the Internet, and everything points to the heart murmurs being the culprit. They are going to run some tests when we get him in early next week.
It's to the point of being able to feel his irregular heartbeat just by holding a hand on his chest. I figured that I would have to be a doctor to be able to detect it, so it never donned on me just to put my hand on his chest and feel it.
He's a great dog, so I hope that we can manage his condition with medication. From what we can tell so far, he could still live a full life, though it doesn't look like he'll be able to run around much, like a little dog likes to do. But we don't always get everything we want. Hopefully a long life will be a good consolation prize.