I state the water is never cycled, since the bacteria do not live on the water column; they need a surface to attach too. What you actually cycle is the filter and that is the reason filter media (i.e. bio-balls) by design has a large surface area.
Nitrates can be removed by water changes or by exporting plant biomass out of the tank. Leaving old leaves, stems, etc. in the tank to rot, will release nutrients the plant took out of the water column, to include nitrates.
To buffer tank safely, can use aragonite in a small bag on the filter. Would start with a tablespoon of aragonite (saltwater aquarium substrate) and check the pH after 24 hours, then adjust the amount as needed

.