What is the difference between aquarium salt and ice cream (rock) salt for use in...

[Hobart]

...tank with fish? Not for a salt water or brackish tank, but just as a sublemental treatment for bacteria/fungus and other such uses of this nature.

[Loollea]

no difference except you pay more if it comes in a small box from the pet shop
you can also use table salt (people salt) if you make sure you buy un-iodized
good luck