No you don't.
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
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....
This salt may contain another additives as anticaking agents nuisible for fish.
Seahorses are salt water and frogs are fresh water, there are special aquariums for salt water fish type creatures.
Salt water is denser than fresh water, hence, it requires more weight to sink in salt water.