I have 6 or 7 turtles that magically appear when the feeder goes off. In my case they are definitely after the feed and they do not scare off the fish nor do the fish seem to pay them much attention. I have actually seen the fish take the feed right in front of the turtle as they are trying to get it. Truthfully the turtles probably do not eat that much as they are rather slow and usually late to the party.
The only detrimental effect from the turtles I have found is that when the water level is high, the turtles will swim over the overflow pipe inlet and get sucked down onto the pipe effectively killing the turtle and stopping water flow out of the pond. This has happen twice in the last 2 months. I have plans to put a screen of some sort on the pipe but I haven't done it yet.
