I use them for two different reasons. I don’t know the entire list of benefits from each but my intention with methylene blue baths and dips are used to treat ammonia burns and cyanide/nitrate poisoning from a long shipping journey. It helps restore the oxygen into the fish’s blood and organs. It will also treat for some fungal infections and can help with some parasites so for me the most important part is that it treats ammonia burns and seems to give the fish a pretty good boost. Being that it’s a good broad spectrum dip is just a plus but it certainly doesn’t get everything.
A fresh water dip I use for two reasons; 1- getting worms to detach from the fish (ie- flukes/ pre PraziPro) & 2- clearing the gills out of built up mucus due to ich and velvet. I have killed a lot of fish from fresh water dips in the beginning of my QT days. Some say that if a fish dies from a freshwater dip then it was too late anyway. Where that may be true I have found a few tricks to be imperative for my success with freshwater dips. 1- kind of obvious but, get the temp the exact same and get ph as close as possible too (Using baking soda). I was doing this before but combined the other two lessons learned and it changed everything. 2- don’t wait until the fish is barely breathing or laying on the bottom of the tank, learn to identify the problems before they get out of control (unfortunately only trial and error gets you this experience and still can catch anyone off guard). 3- This is the biggest fish saver of the three. After a freshwater dip, it’s not uncommon for a fish to go sit at the bottom of the tank while they recover from the dip. Don’t let them do this. If you see this it’s becuase they are having a hard time getting saltwater in their gills and need oxygen. Put a nitrile/latex glove on and very lightly hold the fish in your hand while gliding them through the water back and forth. After a few strides they they will relax and just sit for the ride. When they are good they will swim out of your hand. Stop striding from time to time to give them the option to swim out. If they go back to the bottom and seem to be struggling do it again.