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When I get my new tank how deep (as in front to back) should I get it?

I was thinking about getting a 60" long by 30" high by 30" deep (235 gal.).

But if they both grow to be full size females will that be deep(for them to like turn while there swimming and stuff) and tall( 15" of water) enough?
Or should I get a higher and deeper tank? Like say 48" long by 48" high by 36" deep(360 gal.)?

Help me out here, I don't want a tank bigger then 400 gal. so can some one give me a good size for two full grown female RES that is at least 200 gal. but not over 400 gal??

P.S. you can use this like to see if your size is to big or to small.
http://www.firsttankguide.net/calculator.php

I would fill it almost to the top. Females are supposed to have about 125 gallons each.

Yes, but I have to keep the water level at least 20 in. from the top, because I have a basking dock that floats on the surface. So if one where to stand on its hind legs it might be able to pull it self out. Unless I get a lid.

It would be better to get a tank that's has a greater length and width rather than getting a tank that's shorter but higher. Tanks that are longer and wider but lower give greater horizontal swimming room, which is what you ideally want to your turtles to have.