All depends what kind of corals you want to keep. Acropora in theory should have below 1. montipora, seriatopora , poccilopora around 2-4 . Lps around 5 is ok. It's a theory... that is why we can see some nice tanks with acropora with NO3 10-20 ... there are many other factors like light, supplementation, water movement etc. In my Lps tank I keep NO3 around 5-8 but in sps tank with demanding acropora corals (like Fiji, Australia) I recommend to keep it below 1 ...
Sent from galaxy far far away ...