I'm sure someone more knowledgeable will be able to give some advice, but in the mean time, check your pH from the tap. It might be lower than your tank, in which case, daily water changes at such high volumes might be doing a lot of bad, rather than good.
Your tap water might contain CO2, which then dissipates over 24hrs, once it comes out of the tap. CO2 lowers pH, so as it dissipates, your pH increases. If you're not ageing your water, that means you're putting water in your tank with a lower pH value, so your discus are getting high/low/high/low pH values every day. I hope that makes sense, because fluctuations in pH are really bad for any fish, especially discus.
I might be wrong, but this might be the cause of the problem. If you allow your water to age for 24hrs, it's pH will have already adjusted to your tanks pH, which means your discus won't be subjected to sudden swings in pH value.
Don't do any more water changes without ageing your water. All that means is leaving it in a tub for 24hrs before putting it in the tank. An air stone will age it quicker, but you need to test your aged water, against the water straight from the tap to see if this is the cause of your problems.
Hope that helps and good luck