Performing a Fish Bath
A bath can be performed in as little as 1 quart of water (or even less) or in a 1 gallon Rubbermaid (or similar) container or a small BARE tank.
I generally use a 1 quart container with 1/4 teaspoon of salt and several drops of MB (I also recommend rubber gloves and old towels, rags, paper, etc spread around since Methylene Blue is messy and stains).
The schedule is generally 20-30 minutes at least once per day, twice if the fish will tolerate this frequency.
For freshwater I would add Methylene Blue at double normal tank treatment strength (as per bottle instructions) then add salt (NaCl) at about 1 teaspoon per gallon.
The salt (regular salt; NaCl) can be increased for difficult treatments (such as Columnaris), especially with salt tolerant fish such as livebearers.
It is best to slowly add dissolved salt to increase levels gently in salt amounts over 3 teaspoons per gallon, even in salt tolerant fish.
Generally for most fish (even catfish based on University of Florida studies) 2 teaspoons per gallon can be tolerated for up to 30 minutes (many fish can tolerate 4 teaspoons per gallon), although if unsure about your fish’ tolerance, gradually add the salt via a dissolved solution during the first half of the bath.