If your pH is stable and there is no chlorine or ammonia, then it pretty much has to be parasites. Some like costia are hard to see, even with a good microscope and trained eye. I would treat with Proform C 3 doses. The fish that died probably had some issue, not associated with water or parasites, like heart attack or stroke. Parasites would generally be causing serious flashing and wasting away, or ulcers to kill a fish and both would take longer then that fish took, and have more visible signs of damage.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard