O.K. I did hours of research on this and other sites. Bottom line is you really should not use it unless you really have to. Example: Flame angels... Under stress these angel species are highly susceptible to Brooklynella and Oodinium (Amyloodinium) infections. The former looks like white, clumpy gunk on body and fins, the latter is typical tan to blackish small dusty-dots over the entire fish. In this case I would do this:
do 3 dips of 45 minutes duration every other day.
Using some of the same water the fish came from in a plastic/rubber container add 20 drops of formalin per gallon or if you have a measuring device 1 ml (same as 1 cc) per gallon of water. Also you MUST have an air stone with good aeration in the container during the dip.
I found only one place on the web to purchase formalin: <a href="http://www.petsmart.com" target="_blank">www.petsmart.com</a> of all places! Here is the item: Name: Aquarium Formalin-3 Treatment and Dip, 16 oz
Product Code: 11514
Quantity: 1
Price: $5.19
Again I would only do this on obviously sick fish.
Good luck, Joe.