Exactly the same way you maintain a freshwater tank:
1 - partial water changes
2 - gravel vacuuming
3 - removal of algae
4 - removal of salt creep
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.
no, only if your fish is appearing ill.
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.
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.
You should ask this question in the 'salt water' section.
I have freshwater fish and i use aquarium salt. I put more in every time i do a water change
Good, clean, pure water is needed for an aquarium. Answer You can use tap water, but it must be treated with water conditioner first. I like Seachem Prime as water conditioner.
Nothing...they die....