How many turtles to a tank?

Question
I have had my red eared slider, Bo, for approximately a year and a half now. I just bought him a new tank. I'm not sure how many gallons it holds, but it is very large - 4 ft.(long) , 18"  (wide) and 17" (high)  (Note: The water level is usually 10" high.) I am interested in purchasing another red eared slider to keep Bo's company. I was wondering if my new tank is large enough for Two full grown turtles, and would Bo fight with the new turtle? I think he is a male, incase you need that information. Also, If the tank is large enough, will I have to raise the water level?
Thanks for your Time
Mii

Answer
OK, you have about 35 gallons of water in your tank, and we figure that an adult turtle should have about 10 gallons per inch of shell length. Thus, your tank right now will hold about 3.5" of turtle. Filled up more you could do up to about 5" worth.

An adult male Red-ear Slider is about 9". As a rule, we usually suggest outdoor ponds for adult Red-ears but you can use things like kiddie pools or stock ponds for them.

It often comes as a shock that turtles need so much room, but people seem to forget that they come from wide-open spaces. They always remember their little turtle bowls, and forget that those turtles rarely lived more than a few months.

For more ideas, try http://www.austinsturtlepage.com or http://www.redearslider.com