I think you urgently need some <"floating plants">, they have access to aerial CO2 and can make use of all forms of fixed nitrogen (NH3/NO2/NO3), they will also diffuse light level reaching your submerged plants.I had two cardinals die this morning. I tested my water and ammonia and nitrites zero but nitrates around 100mg/l. My tank is quite well stocked with fish. I need to add more plants
The nitrate reading out my tap is around 18mg/l.
You can get accurate NO3 figures from your water supplier, they have professional analytical kit. Most of the tap water in the S. of the UK has at least ~20ppm NO3. Have a look at @alto's post's, they are a scientist and uses protocols that allow him to <"get accurate results for nitrate testing">.Not sure why the nitrates would be reading that high though.
Personally I would <"carry on using the TNC">, but with lean dosing, until you get some sort of balance.Thanks Darrel. Am I correct in saying that if my nitrates from my tap are high and I have a decent stocking level, that I should just use TNC Lite and stop dosing nitrates and phosphates?