TDS hasn't been mentioned yet.
That is often the silent freshwater tank killer when frequent water changes are not done or if the tank is only topped off.
Fish can be kept with minimal WC's if the filtration/plants are sufficient to handle the bio-load
BUT
that still does not address the increase in total dissolved solids that creeps up, even more so whenever a tank is mainly just "topped off".
That probably accounts for many mystery tank crashes, as has been discussed here before at length.
The other possible issue with infrequent water changes is that if/when a substantial cleaning/change is needed, it can throw the fish into shock and kill them.
How often have we heard
"everything's been fine for years and then I did a big WC and they all died"
The best course of action is at the very least monthly changes of ~20%.
Personally I go ~20% weekly for my salt tank and ~20% bi-weekly for the fresh.
But my little tank with a trigger and two eels gets a 90% change bi-weekly
(I feed them a lot ), but they are tough and can handle it...LOL
The main thing is to develop a habit/pattern that the fish tolerate and learn to expect.
That will vary depending on the type of fish and other specifics that only you will be able to sort out.