Update: Day Three
Baby Blue is bravely tolerating her 'in tank isolation QT' (epsom salt/tannin).
The first day I put her in it, she didn't move, except to come up to the surface for a breath a few times an hour. She adjusted; she doesn't attempt to move much but she knows she's confined. She's eliminating and eating normally.
During her 100% water change tonight, I shone a light on BB in the tannin murk... neither pupil seems to be reacting to light. I will put her in clear water tomorrow, so I can get a better look at her - then, back into the QT she will go.
The corys are having a field day, making the most of BB's confinement! The loach has been loaching around, as usual.
BTW, I finally found sand for my loach - Carbisea's Instant Aquarium Freshwater Sand. They make an extremely fine grained sand which will be great for her. I'm really excited to have found it.
When I bought Laurina, she was a lone loach. Once the corys and the loach are rehomed to a larger tank, I'll check with the aquarium shop where I got her. Maybe they can get a few more from their supplier, so she won't have to be a lone loach anymore :)