Reddit mentions of Kem-Tek KTK-50-0006 Pool and Spa 60-Percent Concentrated Algaecide, 1 Quart

I don't recommend organic hydro. I've tried it to solve exactly the problem you are dealing with and it just creates other problems. For organic hydro to work conditions must be perfect all of the time or else it goes south incredibly fast, since the conditions are also perfect for pathogens to thrive in. If you want to go organic hydro, you need to do a lot of reading up on biofilters and aquaponics. It can be done but it is not easy and requires a lot of attention to detail. For ease of use and results, sterile hydro is best, but there are a few tricks:

1. You need to get an ORP tester. ORP stands for "Oxidation reduction potential" and is a way to measure the effectiveness of the sterilizer in your system. Without this tool you will always be guessing about how much to add. This tool has totally changed the game for me in hydro. You want to keep an ORP between 400-550 to keep bad things at bay.
   
   
2. You need to get some polyquat to kill the rusty looking stuff since once it's established it becomes quite resistant to chlorine. Polyquat 60 is very concentrated - I use 5 DROPS per 50 gallons and only use it when I have scum to get rid of. I had a 50gal res totally covered in red slime algae and after a simple sponge scrub and some polyquat plus 1mL per 5gallons chlorine bleach, res is bright white and water is crystal clear for over 45 days now. Run a higher concentration of both polyquat and chlorine in between cycles to clean your system.
   
   
3. Zone doesn't work. Stick with the bleach, get the ORP pen, and add a few mL of bleach every 2 days to maintain your ORP between 400-550