If a/n/n are reading zero, no immediate need if nitrate reach 20ppm, then a small one would help. What's the tds?
Same here.Using a TDS meter as indication of water quality.
I use 100% RO water and remineralize rather than mixing with tap due to source issues.
After a large water change of 50 to 80% I set my base parameters.
Setting GH, KH and dosing trace, 10ppm NO3, 2ppm PO4 I'll wait 24hrs for the water to mix and stabilize then test TDS.
When TDS values rise about 100ppm above the 'clean' recorded test I do another water change and restart things.
On my NPT (soil based tanks) it's basically the same thing, TDS either stays close to the fresh 'clean' water recorded level or water changes are done when levels rise to remove organic build up.