Thanks for sharing your expeience ..
I think I found the source of problem already..It could be my denitrator again as I can see inside it very cloudy, the cloudiness built up slowly for a week. Though it's working with NO3 measure to below 5ppm from the water dipping output, but the water smell bad. Likely Hydrogen Sulfude already poisoning my tank due to insufficient flow to it.
Now I am confused, it could be the biopellets too which I just introduced last week. Right now I'm running matrix carbon, turn off the denitrator, continue the biopellets. Hopefully water back to norm clarity. Sigh..