You are not supposed to decrease the ISO. If you want to get better pictures at night, you should increase the ISO. You should also reduce the shutter speed because it will take time for the camera to adjust and capture the item or the action that you want the picture to show.
A lot of photographers know that it can be challenging to take amazing photos, especially when there is not enough light, but if you would open the aperture, your chances of having amazing images will surely increase. Having the right equipment will help too.