you have to have basking spot like some drift wood or the TURTLE DOCK.Some water for them to swim, you need plants and fishes for security (optional) and you need a heat to go over the basking spot(UVA and UVB) and you need a filter for it to clean the water but you still have to clean every once and a while.
About a 20 gallon tank.
Depending on the species, one or two. Turtles will keep growing through their entire lives, overcrowding or expecting them to only grow to the size of the tank is something that usually leads to dead turtles.
I think it depends on how big the turtles are.
No.
Tapping on the glass of the tank !
No, a mussel or clam cannot be put in the same tank as a baby turtle. Baby turtles, as well as adult turtles will eat mussels and clams.
The turtles will eat the fish.
You can just use regular water. In the summer/warm weather, I clean my turtles' tank outside with my hose. I also use faucet water. If you'd like to know how I specifically clean my turtles' tank, please contact me or reply to this answer.
Tapping on the glass of the tank !
no RES turtles are fresh water turtles
no they wouldn't
All turtles need a filter in there tank or they will get sick from all the dirt.