I keep a False Map Turtle and Eastern Painted Turtle with my fish in a standard 135g aquarium. The turtles generally leave the fish alone. The painted turtle will chase them a bit if I don't feed him for a while.
The fish and turtles do compete for food. When I feed treats, like earthworms or prawns, the fish often steal from the turtles.
The turtles are both only 4" or so at the moment. They are with a Green Terror (A. rivulatus, female about 6" or 7"), a pair of Yellow Convicts (A. nanoluteus), about 12 yellow convict fry, 5 or 6 clown barbs, 5 or 6 Sterbai Cories, a hoplo catfish, and 3 bristlenose plecos.
I am setting up another tank in which to raise the nanoluteus fry and perhaps breed my plecos, and eventually I would like to get another 6 foot tank. Maybe a 125g as I have to separate the turtles from each other. THey're both males. I have a divider in the tank right now but I'd prefer to get them their own aquaria. I plan to build an outdoor pond for them this spring/summer, and if things go to plan they can each have their own tank (to share with fish) next fall.