don't know how affective thats is. personally my 30 gal get 5ml a day (recently a little more then that because of alga) my 4 gal get about 2 ml a week (but also on that i tend to overdose)
It will only stay in the water colume for ~24 hours.
From: http://www.seachem.com/support/FAQs/FlourishExcel_faq.html Q: Is it better to add Flourish Excel at night or in the morning based upon plant carbon uptake physiology?
A: Carbon intake is a function of photosynthesis. Based on this, it would be ideal to dose Flourish Excel during the day. But given that Flourish Excel can stay complexed as a carbon source for up to 24 hours before it dissipates, you can dose at any time of the day and the product will be available for the next 24 hours. Flourish Excel is not carbon dioxide and there is no impact on pH using Flourish Excel.