I encouraged my daughter to study gut biome but she wants to be an MD and not do lab research for a living. I secretly wished she would do similar research on Reef microbes but that ain't gonna happen.
That aside, these discussions are a bit bothersome in that terms like "good" bacteria vs. "bad" bacteria get tossed about with only anecdotal information about user-reported results with rarely any proper identity of the specific bacterial critter being discussed.
To digress further, gut biome research is a very young science and a fantastically interesting field. Lots to be learned about bacterial roles in guts and in reef tanks. It is a bold new frontier where fecal transplants are gaining validity. Can't make this stuff up.
Someday soon there should be aquarium products available with bacterial strain mixture(s) listed on the bottle ingredients. We have some products now but their application is probably closer to the essential oils market than to a fecal transplant in many instances. I'm glad these products are available now and I trust that improved or better available products will be on the market shortly. It's certainly not all thieves oil and I've seen good results myself but what's in the bottle is still a proprietary mystery.
How often has it been said; "Don't add what you can't test for?"
End of rant. I'm going back into solitary now.