Those turtles also require a good, high-intensity full-spectrum UVA/UVB light (with no glass or plastic between it and the turtles' basking area) as well as a basking heat lamp...and all I can see in that tank is a standard flourescent tube.
You would be best going with a bare-bottom tank, as the gravel/marbles/etc will trap a lot of waste.
As far as plants go, try some ambullia, hornwort, anacharis.
The only problem is that for me, those UVA/UVB lights burn out quick,