On a cycled tank your water prams should be:
pH-vary and fish usually adapt without problems, sudden swings can be deadly
Nitrate-going into the 30ppm and higher can affect the fish immunity, fish can tolerate the slow rise in nitrate and be fine, however, once you make a large water change and the sudden drop in high nitrate you can shock the fish and they will either get really sick or die.
Same as when you add new fish from a low nitrate tank to a high nitrate tank, the new fish are shocked and get sick and/or die and then the old fish with the low immunity get sick and/or die secondary to low immunity from living in a high nitrate water.
This is where the myths from "a water change killed my fish" and "fish store sold me sick fish" come from.
Neglected tanks-lack of water changes, substrate vacuuming, poor water flow from the filter, overfeeding, over crowded etc...a neglected tank usually will have high nitrate
Live active growing plants of the right species and numbers can cause a tank to have 0ppm Nitrate reading due to the plants using ammonia as plant food and the conversion ammonia to nitrite-nitrite to nitrate changes, this is with lots of plants not just a couple.