Tom, thanks for the reply. Well, I have a 10 gallon tank. I use Hagen plant grow fertilizer (5ml/10 gallon) once a week.

Slightly off-topic, however, as Frolicsome_Flora suggested, I should use a drop checker for estimating the CO2 levels. I was thinking of making a DIY drop checker. I hold a PhD in molecular biology and therefore work in a lab where we use ultra-precise intrument and extremely pure chemicals (molecular grade). Now as I understood from some the topics, I have to have a vessel containing NaHCO3 solution in deionized water equivalent of 4dKH. That is 50mg/L NaHCO3. Purity and correct measurement is not my problem. So if 4dKH is 50mg/L, I'll jolly well make this solution. Then I have to add few drops of pH indicator. I could use Bromo thymol blue or Phenol red. And then I have to enclose this solution in a vessel that is made such that the colored solution is in ocntact with aquarium water via a gas phase. Correct me if my understanding is OK.

Thanks.