[APD] kh problems

["James Hogan" <spree_rider at hotmail_com>]

i have a 28gal planted aquarium, Gh 100ppm, ph 6.5, ammonia 0 nitrate0 
phosphate is off the scale and always has been. the tank has been up and 
running for over a year.  my problem is my Kh is always decreasing, every 
water change i add 1tbls of baking soda to the water to be added... my 
tapwater is 0kg 10ppm gh and ph 7.  by the end of the week the kh in my tank 
is down to 0 which is causing my ph to decrease suddenly usually at the end 
of the week about a day before i do a waterchange.  i keep piranha in this 
tank so they are very hardy but are getting stressed out from the rapid 
shift.
does anyone know why this is happening?
and what i can do to correct this problem?

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