Metronidazole kills some protozoan parasites (but is actually originally developed as an anaerobic antibiotic to treat things like clostridium) and is good for clearing gastrointestinal parasites in freshwater that are known to cause unthriftyness and weight loss (spironucleus/cryptobia). Salt water doesn't really have things like this commonly confirmed but it's not absorbed through the GI lumen so you can feed all you want as a shotgun empirical (rule out) treatment. Much more common from wild fish (all tangs at this point are wild except a very few which are collected young and raised in farms) is worm parasites. Metro wont do anything for nematodes or flukes but praziquantel will. Similar to metro, it is safe to soak food in it and feed it out to your hearts delight. If you have the means and the fish has the appetite to do both, all the better. Fast metabolism fish, especially grazers, take months to put the weight back on if not fed constantly but I don't agree that you need to give this tang nori every day...just feed it something 2-3 times a day ideally, once should be no drama though.