Hmm... I honestly don't know enough about this specific information niche to have any concrete thoughts, but I do think it's an interesting idea and I'd definitely like to know more. It's intriguing that a fish would purposefully, perhaps instinctively, eat another fish's poop. I've seen my neons try to eat each other's poop before, but they always spit it out, so it seems more like wishful thinking than any sort of purposeful action.
@Ichthys 's example obviously suggests that some fish do purposefully eat poop, so I'd love to know more about what sorts of bacteria are present, if the species of fish matters (not just their general diet), and how this behavior might manifest in the wild, if it does at all.
There's quite a bit of research on fish gut microbiota, but not really anything that addresses microbiome loss and re-acquisition. Most of what I could find regarding acquisition of microbes focused on the initial acquisition in larvae/juveniles and microbes are usually obtained environmentally, it seems, with exceptions like discus and their weird mucus situation. Couldn't find anything on acquisition of microbes via feces, unfortunately...