For keeping just fish I would recommend a product called Instant Ocean, it is made by Aquarium Systems. If you intend to keep corals you should go with something more like Tropic Marin Sea Salt. Both are available online at www.bigalsonline.com
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.
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.
The best place to find a salt water fish tank is in the Marine Depot. Not only can you find salt water fish tanks, you can also find aquarium supplies and LED lights.
topping up with premixed salt water until you get it up to your target range is the best way
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.
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
Crabs are creatures seeking warmth, keep them in a shaded aquarium, feed them weeds till they can be used for making the crab curry.