I concur with what both Kanundra and Koimaster have said. If Carp wanted to live in salt water they would have evolved to do so and swum to the oceans Millenia ago, but they are adapted to fresh water. It's because we keep them in an enclosed environment that we have to watch the water they swim in.

Salt is a very good remedy for certain ailments and should be used for those alone.

I feed my fish with food that I have added finely crushed garlic and the Sansai Biorex to when necessary, (I also add the Sansai to the pond), that Kanundra recommended, the latter being added for the colder months but also for treatment purposes, whereas I give garlic all year round.

Garlic enters the blood stream of the fish, is extrected via their skin and parasites just don't like the smell or taste. It's not an all over cure for these pests, its a preventative and helps to boost their immune system, as it does ours.

Careful observation of fish behaviour is the best way to catching illness and disease before it gets out of control.

Regards
Ann