Rats — I was hoping that the tail infection that your largest seahorse was battling could be isolated by treating him in a hospital tank and preventing is tankmates from being affected, but if the two smaller seahorses are laying on their sides and have become extremely lethargic, and lost interest in eating, then it appears that they have probably been exposed to the same pathogen. This problem is likely due to a bacterial infection so I think you are doing the right thing by attempting to treat them with antibiotics, Tom, and the combination of kanamycin sulfate and triple sulfa is a good choice if the smaller seahorses are not too far gone…
With regard to mixing the antibiotics you mentioned, let me just say quickly that in my experience both neomycin sulfate and kanamycin sulfate, which are aminoglycoside antibiotics, can be mixed safely with sulfa antibiotics. And I have also combined nifurpirinol with kanamycin or neomycin when I wanted a potent, broad-spectrum antibiotic with good antifungal properties as well as good antibacterial properties. Likewise, kanamycin can be combined with doxycycline to produce a synergistic effect.
However, I would be inclined to be very conservative and err on the side of caution when it comes to combining metronidazole with aminoglycoside antibiotics AND sulfa compounds. That is a combination of medications that I have never experimented with and I can therefore not advise you that they can all be safely used together.
As Yanong cautions in use of Antibiotics and Ornamental Fish Aquaculture, "Combining different antibiotics is generally not recommended. Antibiotics work at many different sites on and in the targeted bacterial cell. Using more than one antibiotic can result in interference between them and, as a worst case scenario, the antibiotics can essentially ‘cancel each other out.’ Most bacterial infections can be treated effectively with a single antibiotic."
So you must be cautious when combining antibiotics. Stick to combinations that you know are safe and produce a synergistic effect when used together (e.g., kanamycin or neomycin plus sulfa drugs or kanamycin + doxycycline). In short, I would not combine metronidazole with the kanamycin sulfate and triple sulfa, sir. If you want to use all of those medications, I would complete the regimen of kanamycin and triple sulfa first and then treat with the metronidazole separately after performing a complete water change.
Metronidazole has mild antibiotic properties but it is primarily an antiparasitic medication, Tom. Metronidazole is an antibiotic with antiprotozoal properties that is very effective in eradicating internal parasites in general (Kaptur, 2004). It is ideal for this because it is rapidly absorbed from the GI tract, has anti-inflammatory effects in the bowel, and was designed specifically to treat protozoal infections and anaerobic bacterial infections by disrupting their DNA (Kaptur, 2004). Since the metronidazole only affects anaerobic bacteria, it is one of the few antibiotics that can safely be used to treat the main tank, since it will not impact the beneficial aerobic bacteria that provide biological filtration.
Good luck treating this latest problem, Tom.