Not really sure about this product, but all available vendor data indicates it's safe for freshwater. I don't see the product name with "marine" in it though, just "Protozin". Are there two different versions?
yes there are 2 types! i also didnt realize!
protozin marine contains copper sulfate 1.7%w/w formaldehyde 0.3%w/w malachite green 0.11%w/w
protozin freshwater contains copper sulfate o.o15%w/w formaldahyde 0.07% w/w malachite green 0.085%w/w