The trouble with higher ISO is that it allows more noise (grain, or what looks like grain). Lower ISOs create the clearest, sharpest, most noise/error-free images. There are essentially two ways to deal with the high-ISO quality problem. One is to simply use a lower ISO, which means you need to put more light on the sensor, whether by opening the aperture wider, leaving the shutter open longer, or adding more light on the subject. The other is to try to get a camera in which the sensor's high-ISO noise problem is as small as you can get, whether due to something about the electronics or the sensor & pixel size as I described above. And when it comes to choosing a new camera based on the sensor, especially in high ISO settings (so you can use a high ISO and introduce as little sensor noise as possible), the recommendation is Fuji.