Many marine fish will show no signs of disease at those nitrate levels, therefore freshwater fish, especially cichlids like midas shouldn't be too bothered by them. I was raised on well water, in a farming community, and kept many central/south/african cichlids that thrived in similar conditions. On occasionally high nitrate levels would rise due to fertilization I assumed. The water was consistently a pH of 7.5/7.6. As long as water changes were done regularly all fish seemed to thrive. I even bred discus and apistogramma agassizii and apistogramma cackatoides (the only fish I ever made a decent profit besides burumundi fronts). I think high nitrates and soft water is where people run into problems. Especially soft water, because it requires a lot of water changes to maintain a constant pH.
On a side note, as others have stated, a large water change that lowers temperatures seems to mimic a big rain and thus spawning season. This trick has been used by corydoras breeders for a long time.
