pH Drops with Water Changes

Question
I have two tanks. A 42 gal hex with a Penquin 200 bio filter and a 12 gal Eclipse.  Both tanks have been set up for about two years.  Large tank has an angel with severe hole in the head (can't bring myself to euthanize) and four panda cats.  Small tank has six tetras. I do 25% water changes & gravel vac weekly and test water parameters with API test kit.  No ammonia or nitrites and nitrates are 40 for the large tank and 10 in small tank. For about the last six months every time I do a water change the pH drops from 7.0 or 7.2 to as low as 6.0 or 6.4.  The water gets cloudy for a couple of days and then clears and pH slowly starts rising.  In both tanks I use salt, Stress Zyme, Stress Coat and Microbe Lift GFCI gel filter innoculant.  I've added some crushed coral to both tanks to help keep pH up.  Tanks have gravel and plastic plants.  San Juan Water in Granite Bay told me the pH out of the tap is 8.2.  What can be making pH drop after water changes?  What can I do to stabilize the pH?  I love my fish but I'm about to give up.  Please help.

Answer
Hi Brenda,
 Hmmm... that is mysterious.  I am unsure as to why you are adding salt to the tank. Neither the angels nor catfish will benefit from that at all -- quite the opposite.  In fact, I would not add any of those things.  I would add only what you need to neutralize the chlorine or chloramine (whichever your water has) and nothing else.  For instance, I have never heard of the Microbe Lift GFCI gel filter innoculant.  What is that supposed to do?    Biofilters are very stable and once they are started you do not need to add anything to keep them working properly -- for example, I have had one running for almost a decade.

-- Ron C.
  rcoleman@cichlidresearch.com
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