i would have to disagree on this one..... pbass are pbass, meaning they all want/need warm water... also from 85-78 is not drastic!!!
IMO... i dont think it was the temperature drop that killed your tem.... 78degrees is still warm!! also a water change of 30% is a little to much but still okay.... normally a 10-15% change will do... but dont worry i sometimes use to do 50% with no problem...lol,, so ill go about and ask you a couple of questions, if you dont mind?....
when you were doing a water change, was that tem right under the new water that your pouring in,?
cuz i notice my tems loved running water they would always jump @ it.... downfall is, if your using tap the water that they were breating was not cured yet... leading to nitrite,nitrate,ammonia,clorine... etc
how big is your tank?
when performing such a big water change.. you dont want to do a big one because you may lose alot of your good biological in that water.. but it also depends on how big your tanks/tank is..
did you feed them before you did a water change?
dont ever do it because it leads to stress for them or any fish...if so, digesting could of been the problem!! either you feed the day before and then water change... or water change and then feed them!!!