Looks like an older, melanistic red ear slider (Trachemys scripta elegans). It's a male turtle. Male red ear sliders sometimes lose all their color as the age (melanistic). They do best when set up in a pond situation. Large turtles like this one are hard to keep in an aquarium because the water gets dirty too quickly. In case you didn't know, water turtles can't swallow food unless they're under water.
Get a large horse trough. Place a cinder block or other large object in the center to act as an island for the turtle to be able to climb up on to get out of the water. Put it in an area that will get sun part of the day, but shade too so the water doesn't get to hot. They eat some veggies or water plants, and insects and animal protein (bits of chicken, etc.).
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I just dump out the water weekly and add clean, but you can buy a filter and set it up more or less permanently.
This one is a 300 gallon trough lined with pond liner because I didn't trust the galvanized material to not leach out toxic stuff:
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That's a "Skippy Filter." You can look it up on the 'net. Very easy to build.