Fresh water dips are a good way to get the gills flushed and get some temporary relief. I’ve found they can be pretty hard on fish when they are too far downhill though. If you do it make sure you glide the fish back and forth through the water to get saltwater flowing through their gills again (It seems this is never mentioned when talking about FW dips and yet it’s one of the most important steps, to get saltwater back into their system...) Otherwise he’ll just go lay down and not recover from the FW dip. If he starts swimming around after you put him back in the tank then there’s no need to glide him back and forth after the FW dip. I’ve found that if a fish doesn’t survive a FW dip then they were pretty close to being past the point of no return anyway. If you think he’s at that point and don’t want to do the FW dip then I would do a methylene blue dip (using the water from the tank). That always seems to help get more oxygen into their system. Do it in a separate container rather than dosing the MB to the tank. Then dose ich attack to the tank. It won’t kill ich, Its just for further relief. Might be enough to get him to start eating again and regain his strength, then start copper or TTM. Otherwise I’d start copper right away after either dip but definitely do a dip as soon as possible (FW preferably or MB as an alternative, they can be combined as well if you want to take it one step further). Doing nothing and hoping he’ll pull through will most likely just prolong the inevitable. The most important thing when a fish stops eating (aside from getting them to eat) is buying time. I've had good results from using methylene blue and ich attack In a pinch. If you don’t have them they are common enough one of your LFS should have them. Otherwise you could get them overnighted. Hopes this helps. It’s always really stressful to watch a fish suffer when you’re not sure which route to go.
Also, you may already know this but if you do a FW dip it is crucial to get the temp and pH as close as possible to the tank water.
