The claim by many Koi keepers is that the water will lack sufficient oxygen at the lower levels, and this insufficiency can be detrimental to your Koi.  The real fact is that if you avoid making your pond any deeper than two feet, there is very little difference in the oxygen levels at the surface and at the bottom of the pond.  the problem with bottom drains is that they have a tendency to promote leaks, possibly leaving your fish land-locked.  now, that’s a problem to avoid at all costs!