Cacatuoids Problem

[Amos@grolen.com]

Hello everyone,

    I have a pair of Cacatuoids, and I have noticed that the male is
having a breathing problem. His gills are moving faster than normal, i
have noticed this since the weekend.  I did a partial water change, that
didn't help. The PH 6.4, Ammonia 0, nitrites 0, There is one other fish
doing the same a Columbian Ram, He is still holding on.  The fishes
color and scales are normal but the fish stay at the bottom. they have
no visible signs of illness other than this. What can I do? What am I
doing wrong?  This tank has been set up since December.
Thank You
Diane