I have managed to keep fish with my 2 turtles for the last 8 months in my 40 gallon breeder tank, although it has been a trial & error scenario !
Predator fish will be better as they are much faster than an ornate fish with long trailing fins. (Although my Turtles have never bothered my Pleco) Just dont spend too much money on your fish as you are likely to be missing it one day.
I have a native themed tank in which i just posted on the forum http://www.tropicalfishkeeping.com/m...anta-ga-56692/
my turtles dont seem to chase the Sunfish now but given half a chance im sure he wouldnt say no to a quick chomp.
Painted turtles are very active and excellent swimmers so it needs more than 8" of water. Its great you have a tall tank, take advantage of that and fill it up. . . . your turtle will love you for it and will act more natural. I wouldnt bother with substrate, as you know turtles are very messy and unless you are prepared to syphon the gravel/sand everyday then i would leave a bare bottom or just place large river rocks so its easy to get the poop out.