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.
salt water
topping up with premixed salt water until you get it up to your target range is the best way
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
No you don't.
they need dechlorinated fresh water, and dechlorinated salt water made with aquarium salt.
You should ask this question in the 'salt water' section.
Sea urchins prefer temperatures at 82 degrees. They can live in salt water temperatures from 72 to 86 degrees though.
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....