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I need a water chemistry lesson

Hi Everyone, 

I have been on the list for a while and appreciate all the effort everyone 
goes through to help others.  I have posted before and received some very 
good info on the water I use for my tanks.  But now something is happening 
that I don't understand.  

Briefly, for a review, my water comes out of the tap at about pH 7.4, it then 
begins to decline to about 4.5 over 4 to 5 days.  Believe or not, the fish 
don't seem to be affected by the water changes or the drop in pH.  All fish 
are soft water species from South America.  I have learned that my water is 
of medium hardness, but has no buffering capacity.  

Given the above, I also have a higher than I should level of PO4 (from the 
tanks' bioload most likely, it is  not from the water supply).  So I decided 
to use Seachems PO4 remover.  It worked great and the PO4 level was set back 
to .5 ppm level with a small amount of the chemicals left in the tank for a 

The question:  When the PO4 remover is left in the tank, my pH is now back to 
6.2 and holding steady.  Does anyone know how the PO4 being removed raised 
the pH?  Here's another one:  When I put in some activated carbon (I do that 
from time to time), the pH continued to climb to 7.4.  I then removed the 
carbon and the PO4 remover and pH began to drop - it hit 5.75 before I put 
the PO4 remover back into the tank.  Two hours later, the pH is now 6.2 and 
steady (no carbon - just the PO4 remover).

I am curious about this.  I have a mated of discus and they have been eating 
the fry (only after they hatch).  They have eaten the last 20 spawns (so far 
only one brood made it to maturity - 37 babies).  I have been trying to 
steady the pH, wondering if that was the problem, and I happened on this 
phenomenon accidently.

Thanks for your help.
Rich D'Ottavio