All turtles but turtles don't like to be handled and aren't ment to be handled,so if ya want a pet you can play with I woudnt recommend a turtle.
No. Not unless you're prepared to take care of all the baby turtles they will have together.
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.
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)
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.
There are a few ways. I only know 2. 1st take the turtles out of the tank (put them in another tank) and get a wind shield wiper against the sides of the cage. OR get a bottom feeding fish. They will eat all the algae in the tank.
Tapping on the glass of the tank !