I used to use the frog before Sundance came out with their version of N2. I used it with dichlor. I emailed them regarding the contents. Its been a while but if my memory serves me they use silver and zinc. It is slightly different than what N2 uses but not terribly different. I believe N2 uses some copper. Whatever it is, it is what allows the frog to use bromine and N2 not to. Lately, it seems the frog has found its niche with bromine users and they market it this way, now.
As for the fish thing, I would lend a guess here and say that in such a small environment such as an aquarium, the silver or zinc could be hazardous to those fish.