It was probably dumb of me to suggest discus in the other thread, but I didn't realize you were 14.
They are great fish, I love them, but they are not too easy to take care of. First off like someone mentioned you need to be sure to get ones that look very healthy. For some fish like most cichlids you seldom get a weak one, and with many fish it always seems like there's one in every batch even if you carefully select only ones that are active and swimming straight and look bright. For discus it's a lot worse than that, I seldom see any that look too great, and never a whole batch that's good at least not in a fish store.
You are going to have problems with them in with so many fish, especially if you stick stuff in and out a lot. Just remember they are delicate.
But the lines on there, no I don't think that's normal. It's also not columnaris unless the guy you bought them from had them, and it shouldn't look like that anyway. It's also not flukes for same reason and other reasons.
So what's the winner? Hole in head.
Hole in head is a constant worry for discus. It's always around to some degree, but it is anaerobic meaning it becomes a serious problem when you have low oxygen. The higher your temp is, the lower the oxygen. You also have a lot of fish in there. Though I am setting myself up for the invariable idiot attack, small fish use up more oxygen on a pound for pound basis so you can't just go by pound as 99% of the people on this site think.
So what to do? First off, just ditch the loaches and add salt. You need to salt that tank, that's all there is to it. Actually, you can get away with some salt with scaleless fish after all I have quite a few bichir in a salted tank, but the problem with loaches is if they roll around on the undissolved salt that's it, it's game over. You can dissolve it first but it's a pain, and if you push it too far the loaches will die anyway.
Second, lower the temperature. Like everything, discus have a livable range - obviously they are outside so it's not ALWAYS 100 degrees outside, even in the amazon. Anything 80 or above ought to be fine. 86 is also really pushing it for the rest of your fish. That is fine short term like to treat an illness, but it is probably going to be too much for day in and out.
Lowering the temperature is going to help with the hole in head because you will have more oxygen absorbable into the water.
Third, do all you can to up that oxygen level.
Fourth, constant water changes. This gets the little protozoa out of there and gives them less to feed on.
So it comes down to hot but not too hot, and plenty of salt and constant water changes, and limit the addition of new fish as every time you do risks your discus and they are both delicate and expensive fish you really don't want to lose. I tried tons of medicine and such but if the evironment problem exists HITH is just impossible to eradicate, as far as I can tell. I'd suggest salt baths too, but you really should not net the discus unless you have to.