Update: the fish appeared more and more miserable as the night progressed, and I noticed the flashing increasing. Fearing he was suffering, and cursing at me under his breath, I made a midnight trip, half hour away, to walmart (I don't even like walmart) to buy water conditioner. Half an hour back, good vacueming, 10 gallons out/in. This morning he is still a bit odd, but not flashing. He's minimally social, but at least he's not sitting on the bottom. The ph is 7.2, the kh is 125 now.
Question - they didn't have seachem prime, but I didn't want to have to wait until Monday to make the change (there aren't any stores open here on Sundays that would have that) so I bought a different product. Ok to use the prime next time? What I mean is, can you switch chemicals? I assume it wouldn't harm the fish?
Thank you!!