yes just add salt!
In freshwater yes. I would not recommend it in saltwater. If it is not in the ocean naturally I would not put it in a saltwater tank. There isn't too much you can't put in freshwater.
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.
A saltwater fish tank can be half freshwater and all of the saltwater fish will live but not for very long.
Yes but you need to buy a saltwater filter to replace the freshwater one.
Certain freshwater fish can survive saltwater conditions; examples are black mollies, mono's and scats. All of these fish aren't truly freshwater or saltwater, they are Brackish. Brackish is between freshwater and saltwater, and so these fish are highly adaptable.
No. Not in the traditional definition of the tropical tank. A tropical tank is a freshwater aquarium. Regal Tangs are saltwater fish and go in a saltwater tank (marine aquarium).
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.
No. Bettas are freshwater fish. Seahorses are saltwater.
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.
It depends on the size of the tank. Ideally most people get 5 gallon buckets of reef salt for ideal tanks.
A plastic tank designed for use with live fish will be perfectly safe. Aside from safety, plastic tanks differ from glass tanks in their durability (generally greater) and their clarity (generally less). Yes they are. Make sure to buy the tank new because older tanks can have problems with leaks.
I use to own a fish only pet store. The saltwater tanks were a nightmare, constant testing, etc. I would recommend you go to a Pet Shop that has salt water fish, preferrably a one owner store. Make sure the saltwater tanks look good to you, clear water, healthy fish and then ask the owner your questions.