How high do nitrates have to be before it will irritate fish? I ask because I gave up on nitrate test kits along time ago because they are so inaccurate. I use the fertilator on APC to guesstimate kno3 dosage. I have good plant growth and it's been a long time since I've had irritated fish. But still, I would wish to find away to verify not having to much build up over time. I dose about 13 ppm kno3 over three days in between 20 % water change everyI use dwarf sag "very common" to monitor nitrates or high nutrients in general. A small amount hidden in a corner to observe is all that's required. If it starts to grow the tank has higher levels of nutrients like ei dosing, If it stays the same or declines then the system is lean on nutrients. This is with flourite substrate, good quality light and c02 injection. glosso also works well for c02 level observation, if it grows well then c02 is good, if it stops or melts down then c02 is on the low side for this plant anyway and could cause problems with other plants soon.
As for fish, most become irritated and chase each other around at high nitrate levels. In general this level is very high and if it's reached over a long period of time it can be extremely high before most fish show much irritation.