150 out of a 0.5 ac lake? Are you running a turle hatchery ;-)? I would say that qualifies as too many turtles. I imagine they eat little bit of everything. I rutinely see one of these turtles hanging on one side of a pellet and a catfish on the other side. Typically it is the catfish trying to take it from the turtle's mouth.
Here is a quote I found on the internet:
http://www.scsc.k12.ar.us/BoubelikB/painted_turtle_information.htm "Painted Turtles can be found sunning themselves on rocks and logs and along the shores of ponds, lakes , and streams. Painted Turtles are omnivorous, eating fish, insects, aquatic plants, fruits and berries. Older Painted Turtles become more herbivorous as they mature.
Because of their size and adaptability to captivity, Painted Turtles are often kept as pets. They have a calm nature and often exhibit comical behavior."
and another one
http://www.herpnet.net/Minnesota-Herpetology/turtles/W.Painted_Turtle.html "Western painted turtles are omnivorous, but because they have a fixed tongue, they can not swallow on land. Obviously, their food must be obtained and eaten in the water. They spend time chasing fishes, aquatic insects, tadpoles, frogs, crayfish, snails, etc. They also consume carrion and will browse on different forms of aquatic vegetation. The taste for the herbivorous portion of their diet is acquired with age; the young are mostly carnivorous. "
