Sounds like it could be a chemical that affected the fishes gills. I agree with the advice above, plus I would run one other filter media, either one will do -
Poly Filter or
Chemi-pure I prefer Poly Filter because it's easier to use. Check out the links to learn more about how it removes harmful organic contaminants that neither activated carbon, algae nor skimming can remove.
crazyreefer, you've got me crazycurious. How did you get away with making only 3 posts in four years!