There are many but Sandstone, Granite, Quartz and some Slate are good provided they have not been contaminated by any kind of chemical or poison.
If you have never set up a freshwater aquarium before, it would be best to call an expert. Check Yelp listings for some freshwater aquarium stores in your area. You will find reviews on them and choose which company is best for you.
Only a very large aquarium would be suitable.
I would add maybe 2 teaspoonsfull of un-iodised/cooking salt to a 10gal tank. I would not waste my money buying aquarium salt for a freshwater set up. Not particularly
Whatever the organism is it will die a dreadfully painfull death.
If a saltwater plant were placed in freshwater aquarium then the plant cells would burst. This is because the salt water would make the plant cell allow more water to come in.
Well i would say they would be nessasary if you want them not to die!
The cells of the saltwater fish would gain too much water.
In most cases it is a tank/aquarium that has been constructed out of materials that could/would deteriorate if it was to contain salt water
I would say yes it would be ok not to put anything at the bottom of the aquarium. Gravel, stone and plastics are usually used for appearance. If it is a freshwater aquarium and if you plan on using live plants then gravel is necessary.If it is a salt water aquarium then it is necessary to use live rock and live stone.
Good choices would be a pea puffer, spotted puffer, or a figure 8 puffer
Not very likely, although if you live in a tropical climate it might ...
Freshwater aquatic snails would be the best choice for a tank that small.