The same as in the ocean
yes just add salt!
Harder than freshwater ones. If you are a beginner, you definitely are better off starting with a freshwater aquarium.
Yes, Blue Tangs (Paracanthurus hepatus), and all tangs, are exclusively marine(saltwater) fish, and will die in Freshwater within an hour.
Algae is a result of high levels of phosphates in the tank. A Phos-Ban Reactor is used in mainly saltwater tanks, but can be used in freshwater tanks also.
Differnt types of fish tanks will need different levels on nitrate. However, most fish tanks benefit from as little nitrate as possible. Saltwater tanks should have almost no nitrates. Freshwater planted tanks conversely, should have about 10-20 ppm nitrate. For most normal, tropical freshwater aquariums, keeping nitrates below 20ppm is acceptable.
In both fresh and saltwater tanks, ammonia needs to be absolutely zero at all times. This extremely toxic compound is produced constantly by fish and will build up very quickly unless steps are taken to remove it. There are bacteria that will remove ammonia and turn it into non-toxic compounds - this process is called the nitrogen cycle. In freshwater tanks these bacteria live mainly in the filter. In saltwater tanks, they are more likely to be found in live rock or live sand, which are the most natural ways to keep a saltwater tank clean.
Damsel fish species include fish like Chromis and Clownfish (like Nemo). They are saltwater fish and if you decide to purchase saltwater fish, please make sure you know how to take care of them. Saltwater tanks must be maintain everyday, especially making sure that water chemistry and temperature is maintained. If you decide to get coral, you must have very strong lighting to simulate sun light. It will get expensive. Budget accordingly. Bigger tanks are easier to maintain than smaller tanks. Water chemistry stays more constant in larger tanks.
I assume by a regular tank, you mean one which contains freshwater fish, such as a goldfish tank. A reef tank consists of a mixed marine environment, containing a mixture of reef fish, corals, invertebrates, and other organisms. With respect to freshwater tanks, there are several types. Ciclid tanks, planted tanks, tropical tanks, and cold-water tanks could all be considered distinct types. Marine tanks come in just as many varieties also. There are cold-water varieties, reef tanks, FOWLR (Fish Only With LiveRock), lagoon tanks, and many others. Hope this helps.
no!
Yes, granite rock can be used in fish tanks as aquarium decorations or as part of the tank's substrate. However, it is important to ensure that the granite is aquarium-safe and has been properly cleaned to remove any potential toxins or pollutants that could harm the fish. Monitoring the water quality regularly is also recommended when using granite in a fish tank.
Forever. The problem is, the water evaporates...and it doesn't take the salt with it. You need a hydrometer to test the specific gravity of your water so you know when to add more freshwater. (That sounds strange, but it's true: freshwater dilutes the saltwater so your fish don't die.)
There is no difference in the physical aquarium itself, only what you put in it. If you go to a pet store and buy a glass or acrylic aquarium, you can use it for salt or freshwater. You may use some different equipment in it based on whether it is fresh or saltwater. For instance protein skimmers are generally used on saltwater tanks only, you use different lighting for freshwater planted aquariums versus salt water coral reef aquariums, etc. Of course the fish or invertebrates are specific to the type of water you are using as well. YOu may see some aquariums advertised as reef or saltwater, but this is just because they are a kit that comes with lighting, filtration, etc. that is suitable for saltwater.