Black spots

First off this post is in the wrong section. Please place question like this in their appropriote section (this would be under Cichlid Central => South American as Discus are Cichlids)

To answer your question, I dont know what a yellow crystal is and I am ASSUMING it is a Pigeon Blood variety (meaning it does not have the black stress bars). Pigeon Blood varieties have black spots known as "peppering"

There are 2 causes for peppering beyond it being a natural thing... (1) The fish is of age to spawn and wants to spawn (2) Your tank is dark, you have a dark substrate/background....

It is natural for Pigeon Bloods to have peppering. Even if neither of the above is true... I have Marlboro Reds and they have peppering on them... its normal... your water parameters (the few you posted) look fine... temp is proper and PH is within an acceptable level. Usually the peppering occurs more to the head of these fish too... also in the fins and less often in the body itself although it can show in the body too...

Dont let Discus stress YOU out... relax, most things you will see are normal... if it always hides, does not eat, has a hole in its head, a bent spine, is not swimming normal, has white spots, pooping white stringy poop or is breathing labored all the time... then you have a problem... usually you are looking for more than one of those problems.... if you have white stringy poop but nothing else... leave it alone, there is nothing wrong... that means you are not feeding a varied enough diet and get some other types of food (white stringy poop alone is more like diarrhea for fish)... but white poop and not eating is a sign of illness...

Point being is Discus ARE picky fish that die easy but THEY DONT die THAT easy! Actually the more I have had them the more I see that they are actually in some ways easier fish to keep than many others, problems and all....