50% could be dangerous, depending on the temperature, quality and parameters of the incoming water and the nature of the set up. How can you convince us it isn't dangerous without providing any sort of information?
50% on my tank would not only have me running out of water over summer, it would most likely lead to fluctuations in my tank water by removing half the already low mineral hardness that buffers the water against the large volume of driftwood. It would also take me about an hour to refill using the garden hose, and would at this time of year cause a fairly big drop in temperature if not pre-heated.