Betta, goldfish.ex.
yes there is a pet fish that you can put in a fishbowl and feed fish food. To get rid of the fish, just drop the fishbowl to break it and kill the fish inside.
Yes, it is bad to put tropical fish in with river fish. The river fish can make the tropical fish sick.
yes if the color of the sand is natural - not painted sand
You can put some but not all cold water fish in tropical tanks for example you can put zebra danios and mountain minnows with tropical if you gradually increase the heat. You cannot do this with goldfish. It depends what fish you have.
yes
Probably the fish. I would not put a crab in a tank with tropical fish unless they were a LOT bigger than it was, and it was not going to grow any larger.
You can't put tropical fish with each other or they will eat eachother
you can put in sea snails and more brightly fish that you cant get with a heater in your tank
a fishbowl
None, goldfish are cold water fish, so having a tropical fish as a tank-mate just wouldn't work because tropical fish need to live in warm water.
No i wouldn't recommend it
You can not put tropical fish in coldwayter bevause they need hot water to survive but you can put goldfish in coldwater becaue they are coldwater fish