Do your Discus have dark bands on them?
I'd be very careful. You could wipe out your tank by raising the pH. At pH below 6, the bacteria that process ammonia "go crazy" (the technical term the biologist who told me this used)
and stop processing ammonia. Also at very low pH, ammonia is not nearly as toxic to the fish as it becomes when the pH goes up.
Use ammonia & nitrite test kits to see where these levels are now. Is is possible that you have very high ammonia levels and that the only thing that is protecting your fish is the fact that the pH is so very low. It is also possible that you have very high nitrate and/or phosphate levels (which Discus don't like).
If you find ANY ammonia/nitrite, I'd suggest the following:
Get some "discus buffer" or "Amazon acidifier" (something to lower pH that is specifically used in Discus tanks that doesn't contain phosphates).
If you can find it, get some Bio-Spira by Marineland or TurboStart #700 from Fritz.
Mix up some fresh dechlorinated water in another tank or even in a large plastic bucket with a heater in it and use the buffer to lower the pH to no more than 5.7 to 5.9. This may be difficult as the pH will lower initially and then tend to rebound upward after a couple of hours depending on the hardness of your water.
Once you have the pH stable in the temporary tank/bucket, put your fish in it.
Remove all the water from your tank and replace with fresh, dechlorinated tap water.
Use more buffer to lower the pH of the new water to the same as the pH of the water in the fishes' temporary home. Watch out for pH rebound here too. When the pH of the fresh water in the tank is stable, put the fish back in the main tank.
Now you can begin to raise the pH slowly on the main tank by doing small partial water changes with dechlorinatred but un-buffered tap water. When you get it above 6.0, put in the Bio-Spira or Turbo-Start to quickly re-establish your bacteria colonies. With the new fresh water in the tank, it will probably take much less tap water to make the pH go up than it used to, so be careful.
Many Discus enthusiasts recommend 50% water changes TWICE A WEEK to keep the fish happy. They really don't like even very low levels or nitrate/phosphate... which is why lots of plants can be so great for a Discus tank, BTW.
In the future when you do partial water changes, you will need to get something like a large plastic trash can and mix up your tap water with the proper amount of "Discus Buffer" to get the pH of the replacement water down to the same as the pH of the tank.... and you will need to do much larger changes than 15% a week to keep the Discus happy.... although if you have a lot of plants, you probably won't need to do two 50% changes a week - as is recommended for bare Discus tanks.
The softness of the water is only an issue if you are trying to breed the Discus. Otherwise, they should be fine in relatively hard (but still acitic) water.
If your Discus had dark bands on them, the bands should be gone a few days after this procedure and the fish should be acting MUCH happier!
Discus are wonderful fish. I love them. But it can be a lot of work to keep them happy.
Best of Luck...