Changing The Substrate

It is very unusual to have zero nitrate. However, you did say you have live plants, and they use ammonia and nitrate as fertiliser so although it is unusual it is not impossible.
Most of us have some in our tap water and the nitrogen cycle makes nitrate. I would be concerned to know that your nitrate remains at zero as that usually means there are no bacteria making it.
Nitrate is only dangerous in the hundreds, though some fish are more sensitive and they need nitrate below 20.

Ammonia and nitrite are the ones that harm fish in only tiny amounts which is why I would have testers for those two rather than nitrate. In fact, I do have ammonia and nitrite testers but I don't have a nitrate tester.

That does make a difference  Zero nitrite is good.

But if things go wrong, ammonia is the first to go up, so you do really need an ammonia tester as well.