Oxygen percentage wise is NOT lower at altitude. The only thing that is difference is the total pressure (ie less mole of oxygen per liter even though the percentage of oxygen to other gasses is still around 21%).
So there certainly wont be any pH issues from it. As an altitude diver the biggest concern is the fact that the pressure difference as you come out of the air is greater than when you dive at sea level.
With that said, these are not things you really need to worry about....aquarius are not that deep, and I have seen no issues with my fishs. Temprature stability can be something to consider depending on your house (we keep our house pretty cold and with led lights need pretty big heaters).