Welcome to the site!
You're wise to do your research BEFORE buying a turtle, it really saves money and frustration to know what you're doing before you get the turtle

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Keep reading through the main site and if you have any questions feel free to ask. My best word of advice is to only start with one turtle. If you do a forum search, time and again you'll see how aggressive RES are and how often they need to be split up. If you can't dedicate one tank per turtle, you should stick with one turtle in the 55g, that's a perfect size for a young turtle into adulthood, it'll last you a while.