Yes, it is bad to put tropical fish in with river fish. The river fish can make the tropical fish sick.
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
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
Betta, goldfish.ex.
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
You can put any other tropical fish that does not have long flowing fins or tail and is not aggresive.
small ones
NO do not put tropical and marine fish together because marine fish have salt water that has to have specific things in it other Wise they will die .but tropical fish just have to have warm water with a heater .you cant mix these together because of the water and ALL your fish will DIE.