Yes -
One of the metals in brass, is copper - this is poisonous to fish. In particular it effects invertebrates (snails, shrimp, prawns, corals), even in trace amounts. There are many compositions of brass but it is around two thirds copper and one third zinc (Zinc is also harmful to aquariums).
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.
Brass, salt water,and air are all homogeneous mixtures.
killing germs.
No, it is not harmful.
Underwater sea animals live in salt water
Both are mixtures
yemo yevadikithelsu
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.
they are like in both they are mixture