As organics accumulate the bacteria break them down and CO2 (carbon dioxide) is released. CO2 also occurs from fish, plant and bacteria respiration. As CO2 builds, it produces carbonic acid which thus lowers the pH. This is all natural, and normally not a problem.
The hardness of the water is also a factor. The carbonate hardness (KH), or Alkalinity, acts as a buffer to keep the pH stable. If the pH is naturally lowering as much as you state, the KH is provably very low.
At this point we need to determine the GH (general hardness) and KH of your tap water. This will tell us how much the pH may be expected to drop. It is also essential to know this if we are to add any type of buffering to keep the H from lowering further.
The GH and KH/Alkalinity you can determine from your water supply folks, they may have a website with data. Once you know the numbers, post them and we can continue.
For some further background and detail, have a read of this article: http://www.tropicalfishkeeping.com/f...quarium-73276/
There may be other factors contributing to the fish loss. How frequent do you do a water change, and how much volume each time?
Are there any fish besides the discus in these tanks, and if yes, which and how many?
What is the temperature?