if white stuff is floating in your turtle tank. you should take your turtle out of your tank.Then you should clean your tank out. Or you can go and by a filter @ united kingdom.Its on (MAINE.ST.THATS IN EAST ORANGE)
Yes they can ! White-spot is a fungal infection - which WILL spread thought the tank !
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
White spots on a starfish usually indicate illness. If the salinity in your tank has changed, it can cause the starfish to become ill and may even be fatal.