I used to keep African cichlid tanks and did not change water for months. My tank water started out with mildly hard tap water of pH 7.6, kH 4, and TDS 100 and gradually degraded to pH<7, kH>10, and TDS>400 over months. In fact, I used parameter change as a timer for my next water change which could be several months apart. My fish were not doing well. While hardy species from Malawi did OK, sensitive species from Tanganyikan often died early from bloat or self imposed starvation. After I changed my habit to do regular WC weekly, there was no more pre matured death. I don't think my fish were negatively impacted by the parameters I measured, but more likely impacted by accumulation of unknown toxic chemicals such as heavy metals and organic compounds that I did not measure.
Not doing regular water change is a time bomb for fish only tank known as old tank symptom. With plants, the dynamic is different as plants are known to uptake heavy metals and organic compounds and as such, have been utilized by wastewater treatment plants to cleanse nutrients.