If lunar gravity reaches the surface of Earth, namely the oceans, then why are there no tides in a fish tank? I mean, the average difference between low tide and high tide is several feet, so how come my tank doesn't regularly overflow? Come to think of it, I don't think small ponds experience tides either. I can remember living by several small ponds (100 sq ft or so) and never noticed any difference in water level no matter what time of day it was. But the question remains: why?