Yeah, it sounds like the new juvenile male did not arrive in the best shape and is having a difficult time recovering. It was a good idea to isolate him and begin treating him promptly.
When it comes to antibiotics, the KanaPlex is a good choice since it includes kanamycin sulfate as the active ingredient, and you can safely treat the seahorse simultaneously with the metronidazole and the kanamycin. Those two medications are compatible.
At this point, I would be more concerned about the pimple-like lesion that has developed on its tail than the negative buoyancy, Sherry. Those suspicious “pimples” are often actually pyogranulatomous cysts, which can be an outward indication of mycobacteriosis, also known as granuloma disease or piscine tuberculosis. That’s a serious affliction and the home hobbyist needs to be aware that it is one of a very few diseases that the aquarist can actually contract from his or her fish if you place your hands in the water and there is a break in the skin (a scratch or scrape or small cut).
So be careful when working in that tank, Sherry, and be extra careful to avoid cross-contamination between the treatment tank and your other aquariums.
Pete Giwojna, Ocean Rider Tech Support