2 gone - water treatment

[JJOZ]

Yikes, 20 to 40 nitrates straight out of the tap is not good and most likely the root cause of your problems, there are products like the ones jeanne suggests but their effectiveness is debatable and it would be a very costly way to keep discus long term.
Fast growing plants can take up nitrates very well but require high lighting and CO2 to really be efficient.
I think an RO unit would be your best bet, these strip everything out and give you the cleanest water possible, so clean that you will probably need to add some things back in to raise the PH back up a bit and restore some minerals that fish require.
An RO unit is easy to set up, I use one for my reef tank but I don't know about what you need to add to RO water to make it OK for Discus, people usually just add some tap water back in but in your case that would mean adding nitrates back in which defeats the purpose.
I'll see what I can find out for you and let you know, but for now I'm afraid you're in a tight spot and I can only recommend to keep testing your water and do water changes how ever often you need to to keep your nitrates as low as possible.
Stuff as many fast growing plants in your tank as you can too.
Try to keep feedings to a minimum and keep your substrate clean.
It sounds like your internal filter is working fine if you have zero ammonia and nitrites but try to replace the filter wool as soon as it starts to look dirty.