Starting a rdwc in 17 gal totes, plan on using 20 gal of ro for my reservior how much hydro guard should i use and often do i add. Plan on changing res about every 10 days. @ Retiredoldguy
Follow the directions on the bottle. I use something much stronger but do it every res change. I have heard using too much can cause a film on roots that can block uptake. So just on fresh changes and maybe midway before the next but havent used it in years.
What are you using instead
I’d bet Southern AG
Southern AG and King Crab(same people who make great white and orca)
For Hydroguard, use 2 ml per gallon of water, so for a 20-gallon reservoir, you’ll need 40 ml. Add it each time you change the reservoir (every 10 days) and after topping off with significant amounts of water. Keeps roots healthy! 
