I actually just quarantined a fish today that has a sunken belly, and began treating it with metronidazole. Do you think your stuff would work better?
Metronidazoles primary use is for the treatment of anaerobic gram positive and some negative bacteria including those that produce beta-lactamase. I've seen that Metronidazole is sometimes effective for protozoan parasite infections (icy, heximita, etc). It's also sometimes effective for some internal parasites such as nematodes. Some internal parasites it's not effective for include camallanus worms.
Fenbendazole will treat internal parasites including camallanus worms. Fenbendazole is also considered a safer alternative to another powerful de-wormer, Levamisole. It will also treat planaria and hydra as previously mentioned.
"Working better" will depend on what it really is. If it's bacterial at all (though I don't think so), metro would be a better choice. If it's internal parasites, I think Fenbendazole would be better so long as your fish will accept foods. It needs to be ingested to work well.
Also, if unaffected fish are treated, would it harm them? I was thinking of treating the whole tank in case it spread but wasn't sure.
I don't think it would harm healthy fish. As a precaution I've been treating all of my tanks with fish larger than 2" (I'm mixing it with 1mm NLS - not sure it'd be effective to feed to fish which can't ingest an entire pellet). For treatment I'm feeding treated food for 3 days straight, then wait 10 days and repeat treatment. When treating, it's suggested to syphon out the fish waste and potentially ejected worms/parasites daily.
