You mean this stuff ?!!
There are several different ways to carbon dose safely and cheaply.
I vodka dosed a reef tank for over a year and now am in the process of starting vinegar dosing on my newly moved reef. I gave the vitamin C a shot a couple years ago but I don't think I understood it completely back then. I used that product in picture [still have it].
Most in reefs use these tools for nitrate and some PO4 reduction with focus on nitrate. Most fresh water people have only looked at these same product as a nitrate reduction tool instead of a plant 'supplement' . Most just conceited that water changes were easier then dosing a small amount like 15ml per 100g once or divided into twice a day.. ?
Like I said before I am not the plant guy but I still use many of the same tools in different applications . The dosing is an easy learning curve and I can link you what the reefers do with vinegar...
Kind of funny to me after you find out some add vinegar [or vodka] to their tanks daily when others are nervous they didn't rinse their new tank well enough and the may be some 'residue' !