Great looking tank. Thought I would chime in since you are thinking about adding a turtle and I have kept many aquatic species over the years...
In all honesty, I would advise against it. It can and has been done but I find it just becomes a hassle from past, personal experience. As stated above, regardless of the species of turtle you will need to provide it with a basking area (a spot where it can get completely out of the water) as well as some form of UVB lighting. You then also risk the chance of having the turtle nip and or eat the fish. Some species are better than others with fish but they all have the potential in seeing them as food. Turtles are also VERY messy and destructive and will add a huge amount to your bioload.
Good luck with whatever you decide and feel free to ask me any questions!
In all honesty, I would advise against it. It can and has been done but I find it just becomes a hassle from past, personal experience. As stated above, regardless of the species of turtle you will need to provide it with a basking area (a spot where it can get completely out of the water) as well as some form of UVB lighting. You then also risk the chance of having the turtle nip and or eat the fish. Some species are better than others with fish but they all have the potential in seeing them as food. Turtles are also VERY messy and destructive and will add a huge amount to your bioload.
Good luck with whatever you decide and feel free to ask me any questions!