Their tank looked fine, but had hidden problems in the gravel. I stirred up just a little bit of gravel, and in seconds the tank went from crystal clear to this:
I'm not going to stop trying, however. Next time, I'm going to use no gravel, and have protected overflow that drains into another chamber that cleans the water with great detail.
The jellies are alive! Maybe 3/4 of the 8 of them can be saved. I'm already down to 8 jellies, and I started with 11.
If I have one thing to say, I would tell you all to make sure you change your water even if the water seems to be fine. Thanks for reading.