Johnny, I wouldn't kick yourself in the foot and 100% blame the large water change or yourself. Take all the advice and recommendations from us with moderation. There is always a chance everything you hear here might not be the case for you at all. I've been through hell doing everything single thing I could to my best advice and and months later there is still no proof of what exactly went wrong. There are theories and strong pieces of evidence to suggest multiple faults, but the fact of the matter is you will never know.
The best bet you have is to take the wisdom from the wonderful people here and apply it moderately to the best of your ability. And trust your instincts.
While I myself used to do 90% pwc daily, the next time I get discus I plan to do a maximum of 50% and that's every other day. The truth of the matter I've learned is that a stable PH will come a long way. That's the one thing I've learned that all my advice I've been given has connected to.
In my case I consider the low KH to have affected my PH, and overtime the stress became too much without me realizing it. Reading through your posts, I would highly recommend an aging barrel and reducing your water changes to 50% or less. For the first couple days to a few weeks, I would check your PH, KH, perhaps even chlorine levels of your tap everyday. Trust your instincts. If you feel chlorine is an issue, test for it. If you feel PH is an issue like I did, test for it. Be relentless in your testing for a minute. You can let off once you've gotten it down so don't get intimidated by the tediousness of testing a lot. It's just until you get everything down to a science.
And remember, don't 100% bet on a specific thing you hear here, always keep your mind open and trust your instincts to a fault.