Did you mean Iching? Itching is a side effect of dry skin, salt can cause dry skin. Reducing the amount of salt can reduce the effects. Ich is a fungal infection that usually affects fish in freshwater tanks as salt kills infections. A way to prevent this is to add a solution to the water in your tank that kills Ich (ask the people at a pet store).
You can raise a sea urchin in a salt-water aquarium. There are no fresh water sea urchins so you would need a salt-water aquarium to properly care for the urchin.
killing germs.
Aquarium salt is manufactured for different reasons than making your aquarium the perfect environment for salt water fishes. You may use it in an emergency, but long term is harmful to your fish. Marine salts contain the necessary salts and minerals that fish need are the best salt to use.
Underwater sea animals live in salt water
yes, in small doses it can be used to prevent infection.
No you don't.
The best treatment is clean water and you may add up to 1tsp of aquarium / kosher salt per 5 gallons of water to prevent bacterial growth.
they need dechlorinated fresh water, and dechlorinated salt water made with aquarium salt.
You should ask this question in the 'salt water' section.
Regular salt may contain anticaking agents nuisible for fish.
I have freshwater fish and i use aquarium salt. I put more in every time i do a water change
Nothing...they die....