I have worked closely together with a big discus breeder/seller. Large waterchanges was all he did, and stil does now he is in America. And healthy feeding.
Luckily if you want to keep plants you can also do large waterchanges. If you want to breed succesfully in a planted tank that will be tougher, the eggs need low osmotic values to be fertilized solidly, and a planted tank needs ferts to keep the plants happy.
But the discus can adjust to most watervalues, exept they prefer a bit higher temperatures.
At that discus setup we never did ANY tests. Good filtration and large waterchanges, that is the secret.