Since MES is a biological buffer used in plant tissue culture then
I can't see any problems with using it. But, with Discus I don't
know if I'd be willing to risk it with such expensive fish.
If you do decide to use it, I would suggest you try it
in a plant-only tank to start with and then maybe introduce some
"expendible" fish such as feeder guppies. (Sorry if I offend
people by considering some fish expendible). Of course, these
fish would need to be acclimatized to a low pH tank first so that
you weren't killing them by pH shock.
I think Seachem has a non-phosphate low-pH buffer that works quite
well. I bought some when it first came out, but there were some
problems with cloudiness if you combined the acid buffer & their
neutral buffer (as the container suggested for pH around 7.0).
They then made a newer product which is purplish colored & this
works very well for me. It's not cheap though!