I also initially questioned the accuracy of the test results which is why I also tested store bought bottled water and distilled water. Finding they were both zero for nitrates suggested that my well water and tank water are very high in nitrates. We could quibble over absolute values, but it wouldn't change the fact that based on comparative analysis the nitrates are way high.
It's generally held that aquarium nitrates should be less than 40ppm, preferably less than 20ppm. At some point, high nitrates causes problems - not as short term deadly as ammonia and nitrites, but general health and longevity. I too have had numerous fish losses that I could not explain. A couple recently due to dropsy or constipation. Of the first brood of some 20 Red Wag Platy's we raised, only two remain and before they passed, most showed signs of stunted growth.
While some fish seem okay, others are lethargic and rest on the bottom or at the surface much of the time. I have a couple right now showing signs of shimmy. Of course, I can't be certain that the high nitrates are the root cause, but now that I know there's a water quality issue, I need to take steps to resolve it.
I have read that by themselves, plants prevent nitrates better than removing them by using the ammonia...so nitrites and nitrates are not created. Plants can process nitrates, but in an environment where ammonia exists (like the aquarium) it will use ammonia instead as it requires more energy to process nitrates.
So plants would help by otherwise reducing tank generated nitrates, but likely not solve the overall source nitrate water problem.
When you research lowering nitrates in an aquarium, the #1 solution is the water change - increased frequency or volume. But of course this is the last thing we should do if the source water is high in nitrates.
We would either need to use other water for partial water changes and/or find a way to keep nitrates in the tank low. Water is about $1 a gallon. I was doing a 30g water change on my 60g tank weekly. Even if I reduce to a 10g weekly change, it adds up. API makes a $50 tap water filter for nitrates (which I may also get), but the filter cartridges are expensive and some reviews suggest that the cartridges exhaust very quickly. It has to produce 50g+ to break even (over purchased water).
API also makes Nitra Zorb which is yet another synthetic adsorbant that removes ammonia, nitrite and nitrate. It can be recharged in salt water for numerous re-use (unclear how many times). I'm hesitant to use this as it upsets or circumvents the beneficial bio-filtration.
Then there is the dedicated bio/nitrate filter on the tank.
The obvious solution to this problem is to remove nitrates from my well water...but shooting the farmer or moving just doesn't seem practical
and a house system for nitrate removal is expensive. Although I'm usually against shot gun solutions, I think a combination of nitrate reduction methods may be the answer and may allow a prudent water change with my well water.