@Randy Holmes-Farley it's me again with more question .. I hope my questions don't bore you. I did some further readings and it seems that there are: Correlation between ULNS (No3 and Po4) and Alkalinity. The lower No3 and Po4, the lower Alk we should be shooting for (7.7 dKH to 8.5 dKH). The higher No3 and Po4, the higher Alk we should be shooting for (> 11 dKH). Correlation between Calcium and Alkalinity. ........... 8.25 dKH Alk to 420 ppm Ca. 8.50 dKH Alk to 422 ppm Ca. 8.75 dKH Alk to 423 ppm Ca. And so on. Whilst I understand matching the seawater parameter at 1,290 ppm (Mg), 8.5 dKH (Alk) & 420 ppm (Ca) at 1.026 SG is recommended but I failed to understand the above. Does it mean that higher No3 and Po4 tank should be shooting for >11 dKH of Alkalinity, hence to balance that, Calcium will need to be at 439 ppm? And I'm not getting the correlation between Magnesium to all of these parameters (No3, Po4, Alk & Ca). I'm pretty sure I'm confused and probably there is no need to know such intrinsic details.