were you getting any bacterial blooms when you dosed nopox? Just like [MENTION=6904]Zoarder[/MENTION] the amount of rock or lack thereof seems to be the elephant in the room. denitrifying bacteria primarily exist in anaerobic areas typically deep within porous rock, deep sand beds, or blocks. Even adding in nopox the bacteria might not have enough surface area to survive and other bacteria will feed off the carbon which leads to blooms and cloudy water. If you don't want to add rock then try adding more blocks in lower flow areas of the sump. some other hobbiests have pretty good setup designs with egg crate and siporax.