Weather or not a turtle will go after fish will depend on the species and how old it is.
Most aquatic turtles are mainly herbivores as adults.
I have about 8 or 9 different species of turtles in tanks with fish and they all leave the fish alone
but it would not surprise me if I woke up one day and some of the fish were missing. :'(
They are pretty lazy so as long as you provide them with a different easily available source
of food they will usually leave the fish alone.
I can't believe your red eared sliders leave your plants alone.
In a couple of hours mine would have that pond looking like you put the plants through a wood chipper
If anyone is interested in getting a red eared slider for there pond please get it from a reptile rescue and not from a pet store. There are so many unwanted ones out there looking for homes. :'(
I got into ponding last year when I built my pond to house some of my turtles during the summer.
If you do get a turtle make sure the pond area is escape proof, as they tend to wander off if you give them a chance.
Here is a picture of some of mine from last year.