No, your turtle will not be "OK" without a heat source for bask i.e. the log you put in the tank for it to "hang out." Reptiles are cold blooded. For a painted, basking temps should be 82-88F. You will need to purchase a UVB so the turtle can utalize the Vit. D3 for calcium absorbtion. Next to a window does not work. Glass acts a prism and also filters out about 30% of the natural sunlight. Thus, the nessessity of a UVB bulb so it does not get Metabolic Bone Disease. (MBD)

Also, there is a strong possibility the turtle will eat your fish, or try too. You should release it back where you found it, or set up a different terrarium if you want to keep the painted long term.