Well, the Maracyn worked. The protrusion on Vishnu's eye went away and the coloration around his gills and lower jaw is now consistent with his radiant blue color.

An interesting note. Yesterday, I did a 100% water change, rinsed the Anubias, heavily rinsed the gravel, returned gravel, stones, and plant to tank, with new water (which contained Fe, K, and Micronutrient plant fertilizers), and put Vishnu back in. This morning, I noticed that Vishnu had super-fine threads trailing from his fins...all of them. He otherwise looked energetic and happy, but those threads worried me. So, I dosed with Maracyn again. Within an hour, the threads were gone.

I wonder if they were a result of one or more of the fertilizers. The conditioned water assists with a fish's protective slime coat. I don't see how brand new water could yield such threads in just 24 hours.

Oh, well, better safe than sorry. I may severely cut back the fertilizers, continue dosing for a few days, and then use warmer water when I do a change. Betta's just don't like water under about 74 F, which is hard to maintain in a 0.5G unheated tank in an apartment that's never above 68 F. The little 7W bulb helps, but not enough.