Know your chemistry.
Chloride (Cl-) is not necessarily bad or even unhealthy for fish. It is an ion that is VERY common in salts (even aquarium salt).
Magnesium and sodium both have very common chlorides that are found in water (especially salt water).
Sodium chloride is table salt.
A little bit isn't going to hurt freshwater fish.
Hell, alot of people suggest adding it (under various other names) to the aquarium to combat all sorts of illnesses from fungus to ick.