When I first began fishkeeping, I knew little and reading all the rhetoric about which fish prefer which PH was confusing so naturally, I asked the LFS for advice and they sold me PH Down and PH Up. I eagerly added one (I forget now which direction it was now as it was many years ago) thinking 'I was optimizing their water conditions' and 25 fish, all but one had perished overnight. A sudden or drastic PH change is almost always fatal or does so much damage, they don't live very long.
If you give most any fish any PH SLOWLY, they'll adapt and thrive in it. Yes, optimal PH has virtues, especially species-dependent virtues but a stable PH is imperative.
If you know you have an established nitrogen cycle, and you know the water is treated, and you measure the parameters weekly or whenever there's reason to suspect they may have changed, just allow them to adapt to your PH. NEVER allow PH to vary more than 0.3/day. Less is even better but I've had no issues with 0.3/day. I once did a large WC and it moved 0.5 and they were all visibly very uncomfortable and breathing rapidly and one parrot fish died. Frankly, I was just lucky so I'm much more careful now. Don't feel badly. We've all made similar mistakes with similar tragedies when starting.