Some of our friends in Singapore have had water problems of late in their tap water which makes me wonder if there may not be a climate condition causing many lakes in the geographic region to be in a turnover cycle. That would explain a temporary nitrate rise. If this has been a long term issue (judging by your preliminary test reports it seems that is the case) it could possibly be a natural mineral source in the local aquifer causing the problem.
Either way it sounds as though you have things well in hand

.
Thanks again for your unbiased approach to this experiment.