You're wise to research this before adding turtles. Turtles have been known to damage pond liners with their claws, so that is a valid concern. It doesn't happen often, but it is a possibility.
There are bigger problems, however. Small painted turtles don't stay small. As they grow, they will eat your fish. Even as babies, they are likely to chase them and damage them. Fish are one of their natural foods. The fish pose a danger tot he turtles, too. Domestic fish often carry bacteria which don't hurt them, but will make a turtle sick. As the turtles grow, they will produce a tremendous amount of waste, making it difficult for your filter to keep up and maintain clean water.
People who keep turtles in ponds usually have only turtles, and the ponds are especially designed for the turtles. Turtles are somewhat difficult to keep in small ponds. Hibernation is a dangerous time, even for wild turtles who have adapted to outdoors and can find the optimal places for hibernation. For domestic turtles in man-made ponds, the risks are even greater. Turtles can suffocate, starve if they don't have enough fat built up, develop fatal bacterial infections if they eat too close to hibernation time, and be killed by toxins that build up in the water. There is no way to absolutely ensure that turtles will survive the winter. There are ways to increase the chances that he will. There must be at least one foot of unfrozen water at the bottom of the pond. There must be something to dig in on the bottom, so an earth-filled box would work. The water must have adequate oxygen in it - turtles absorb it through their skin when hibernating. It's a good idea to have an electric de-icer on the pond's surface to allow oxygen in and the detrimental gases out. The following site has more details on turtle ponds:http://www.fishpondinfo.com/turtles/turpond.htm
Turtles also must be properly conditioned for hibernation, and should be examined by a reptile vet in the fall to be sure they're fit to hibernate. There's more about that here:http://happyturtle.ms11.net/hibernation.html
The time leading up to hibernation is also risky. If you have an extended period of damp weather with temperatures in the 40's to 60's, the turtle will become less active, but it's not cold enough to trigger hibernation. Skin and shell infections and respiratory infections are common in those circumstances.
Besides hibernation, there are a couple of other problems you may encounter with outdoor housing. One is that a turtle is likely to simply leave. They go off seeking mates the first chance they get. A small fence a few feet out from the pond will be needed. Many people make a rock or concrete block wall just tall enough to keep in a turtle. Predators often injure or kill turtles that live outdoors. Raccoons are the most common offenders, and they will chew off feet, and destroy shells. Dogs will grab a basking turtle and carry it off, thinking it's a toy. The site I gave you above on pond design has some tips to minimize these problems.
My recommendation to you would be to stick to the fish and frogs, unless you want to go to a lot of additional trouble for turtles. At some point, you may want to consider constructing a turtle pond. The site I gave you above will help if do that.
If you have more questions, just let me know by clicking on REPLY. I'm not trying to discourage you, but just want you to be aware of potential problems so you can avoid them.
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