Whoa! I'm not the expert, but I think that ph 8 is way too high for those plants. I think Amazon rain water naturally has ph of like 6 or even 5. Remember, each degree of ph is exponentially more or less acidic/basic, so even 2-3 points on the scale Is quite a difference (10^3=1000x ph).
I would take off dead or weak leaves ( if any have any sign of life, put in some sealed container with sterile media and try to get some clones)...
Are there any healthy leaves left on the plant? If so, I'd spray/mist with a citric acid solution every 2 days (this will kill any bugs/mites on contact, but also the runnoff will bring ur ph back down).
If it's looking reall scary with no leaves no new ones coming, I'd uproot and put in a sealed jar or something with just enough water to cover roots halfway (contact with air and water) add like 5% h2o2, switch the water out everyday, twice if you have time. This is like a super ghetto , manual cloning jar, and should be pretty clean an humid, I call it " the infirmary", and I've ressurrected many goners this way...
Best of luck!