Did you notice a temp. change in your aquairum when you did the water change? If the temp changes too much it will shock the fish and can kill them; frequently such death is not immediate but somewhat delayed. While ph change can be harmful it has to be pretty sudden and pretty extreme. I been pretty heavy handed with my aquairum this year (using ro water in aquariium with tap water for example) and really none of my fishes have reacted badly (i have wc b. cupido, wc discus, wc geo, ...); the discus are the most sensitive to changes and definitely show a deep hate for more than 1.5 degree change in temp during a water change. Truth be told the b. cupido and geo are pretty robust. I do have 14 angels with some krobia but they are overly robust and near adult size now so i can't really compare. Young fishes are more sensitve and delicate but quarter size actually have some decent age on them and not that fragile (dime size are a bit fragile); most of my fishes are dwarf cichild with about 2/3 in black water but i woudln't call them delicate or fragile as long as the water chemistry is pretty constant.