Based on your calculations, what you have is a magnitude 8 spring, most likely a Fracture spring. Springs of higher magnitude would supply enough flow rate to totally support the creation of a stream connecting the two ponds. Yours will need a pump to get an acceptable flow rate for a stream. Is the lower pond located on a slope or is it at the bottom of the slope? I would recommend a minimum depth of 4 or 5 feet. Have the water tested before you add fish. Sadly, most of the water in this country is contaminated to some level by all sorts of toxins. You don't want to buy Koi just to kill them. I would rule out underwater lights. Koi are natural bottom-feeders. They will keep the water stirred up and turbid most, if not all, of the time. You will want to use liner in your stream to prevent erosion, but evidently neither of the ponds will need liners. More to come as info is provided.