Hi @Lasse - Sorry - it was about a 2-3 week period between Nitrate measurements - not a couple days. No I didnt measure nitrites. At that point there was also no indication of a problem - except a huge piece of GSP that I have had over 10 years was not opening. After a week - nearly all of the acropora had RTN'd. Also did several large water changes - and the nitrate fell to the appropriate level - and didnt rise again (i.e. 50% water change - level dropped 50% - another 50% water change - the nitrate level dropped to 25% etc). The GSP did not open for 3 months - now is coming back.@MnFish1 In order to understand more - did you measured the nitrite when you get high nitrate readings? Rise of nitrate from 5 to > 120 ppm during a couple of days indicate that the nitrification process is in disorder and you have high nitrites instead.
Sincerely Lasse