At any rate, I figured adding the medicine tablet couldn't hurt if they were already dying. (Although I knew it would kill all the snails, and I rather like it when I have a controlled number of snails in the tank.) I removed the filter bag per instructions and put the fizzy tab in. The improvement was almost instant. The medicine colored the water bluish, but Bob's eyes cleared up.
I left them like that overnight, but in the morning on Sunday Bob's eyes looked a bit cloudy again. I couldn't add more medicine for three more days, so I decided to really clean the beejeezus out of the tank. I probably did close to a 100% water change, and as I was doing the third "flush" out I noticed that there WAS a LOT of gunk coming up. What's strange to me is that this tank (the 10 gallon one) is the one I usually consider the cleanest. I am religious about changing nearly a third of its water every week. I sometimes miss the one upstairs and the big tank is hard to do more than 10%.... so I thought I'd been keeping up on this tank. My suspicion is that the java fern, which has nearly taken over the entire tank, may have been hiding bits of detrius -- food, waste, rotten eggs (from the multiple spawnings of the minnows), and any number of other ugliness.
After that big clean-up, I also added a secondary filter (I had a 2 gallon mini-canister filter around from the old betta tank that used to be in Mason's room), thinking that might help improve the water quality. I'm also planning to do daily changes until I can't get any more of that white gunky stuff up.
Bob seems to be appreciating my efforts. This morning his eyes were bright, and both he and his minnow collegues have been super zippy (and, well, alive at any rate).
I'm crossing my fingers that this fishy disaster has been averted. Is it too soon to call it the Easter miracle, I wonder? :-)
Oh, and what the hey: Amazon Fail!? Sheesh.