1.) I agree with the advice to pull water from depth to provide some degree of aeration to the pond.
If you know the depth of your epilimnion (the top layer of oxygenated water) during the summer then I see two options. You could put your pipe intake a foot or two above that depth, and increase the oxygen content in the layer that the fish are using. Or you could make your pipe intake a foot or two deeper into the lower oxygen waters and perhaps expand your usable water depth to the fish. (I would probably go with the first option, and provide more oxygen in the layer the fish are using, to slightly minimize your risk of a summer fish kill.)
2.) I almost never disagree with Theo (he is an engineer and I am not), but I do not believe your pump is "lifting" water from depth. The rule of thumb requiring more energy to lift water means to lift it against gravity. For example a water well supplying 100 GPM with a producing water level of 300' below the surface will require much more energy than a 100 GPM flow with a producing water level of only 100' below the ground surface.
That is not the analogy to your pond fountain. Your 15' extension tube will fill exactly to the level of the surface of the pond, so you are NOT lifting any water to the intake of the fountain pump. Your only additional energy requirement should be the very small frictional pressure losses due to moving your fluid through 15' of pipe. I think a bigger problem might be changing the net buoyancy of a floating type fountain.
3.) I would be safe with a GFI in the wiring loop. However, I would be extra safe and have a pond rule that no one can wade or swim unless the fountain is turned off! 120V AC electricity and water do not mix.
