Hi Martin and Welcome to TFF!
Sounds like a really nice big tank to be planning! 212L/56G 36x15x24 inches. If its not yet filled with water you have a golden opportunity to take your time and discuss all the hardware with the members and perhaps optimize a few things prior to beginning to deal with water chemistry. That's a pretty rare thing around here as most have already begun some things and then come here when things first go wrong! Be sure and put your location into your profile as that can further help the members answer questions (assuming you don't mind if it appears there.)
Sorry, but I don't know the answer about turtle that are completely aquatic and whether any might stay small enough for your tank and also whether any might be able to be raised along with fish! Those are interesting questions and I'll be as interested as you if someone among our members can tell us more about this. I imagine there are pet shops that get questions like that pretty often and some of our members are among the retail industry I believe. This particular beginner's forum revolves mostly around freshwater tropical fish tanks, but we enjoy all sorts of discussion.
If you are truly new to the hobby, there's a lot to absorb surrounding the startup of a freshwater tank. There's the choices of equipment to go along with the tank. There's the sizing of the equipment. There's the learning about filter media. Then come questions about tank startup itself. Once the point is reached where water goes in then there's lots of learning about water chemistry and finally there's planning for the new occupants all the while.
Among the things often missed by beginners are an understanding of what and how many fish their tank is likely to handle and what type of water test kits they should have to get tehmselves started. As mentioned another bit of learning is about what filters are, their purpose and major functions and how to chose the right materials for the desired functiions. When not asking questions and reading answers here, be sure to be reading in our Beginners Resource Center - lots of good info there. Start with the Nitrogen Cycle, Tank Startup, Fishless Cycling and Fish-In Cycling. And be sure to read and participate in other members threads. That's usually the most fun way to learn and people always enjoy hearing another voice.
Good luck!
~~waterdrop~~
