And the follow-up.
Weekly water change and trimming performed.
Staghorn was slightly reduced. I didn't expect much, since its a red algae like BBA, which is considered resistant to AlgaeFix.
The guppy that was severely affected continues to do well, apparently no worse for the wear.
But another guppy that took longer to stop gulping at the surface than most has developed dropsy. This is an extremely rare occurrence in my tank keeping history, so it's hard to write this off as coincidental. It's currently being treated with Kanamycin medicated flake.