Did you test for ammonia? If the pH was low and there was ammonia present it would not have bothered the fish. When the pH went up (clean water added), the ammonia would have rapidly become toxic. And caused the behavior you describe. Also, check your tap water to see if it has ammonia. If you have chloramines, it is possible you did not use enough water conditioner. I would dose with Prime and do another major water change (if your tap water test out ok). Our water here occasionally gets so bad that I have to skip water changes, you can smell the chlorine and crap when you run the water. I am told that they occasionally "flush the system" and whatever it is they use it seems to be hard on fish. In addition to the Prime, do you any carbon in your filter? I would use some new high grade carbon or some sort of resin like chemi-pure for a while to make sure you are getting any possible contaminent out. I just noticed you are in Haubstadt, I used to go to races there. I am also in Southern Indiana, but a different part. I bet your tap water is probably not a lot better than ours is.