Root problems and slime balls

[AstroDon]

I recently built my first hydroponics set up, an NFT system growing strawberries. I had a problem when I first put the bare root strawberry plants in the system, all the roots turned black. I'm not entirely sure of what caused the roots to turn black but I had a large air stone in the reservoir and later added some H2O2. I also changed the water/nutes and flushed the system with new water. I use my well water which measures only 49ppm on a TDS meter (900ppm after nutes), and I maintain a 6.0pH and water stays right around 75.6°F. I have been using nutrients for strawberries that I bought on ebay, there isn't much info about them online but the bag says "Urban-Hydroponic Strawberry 24-12-32" and gives directions for how much to use for seedlings and mature plants (800ppm and 1200ppm). Heres the Amazon link for the nutes- https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

When I ordered the bare root strawberry plants (3 different day-neutral strains- Albion, Seascape and Mara De Bois) I was still building the NFT system and getting the room ready. When they arrived I wasn't ready to put the plants in the system yet, so I put them in the refrigerator for about 6 days until I had everything working. I'm thinking that the roots turning black was because of their stay in the fridge, I kept close watch on them making sure they didn't dry out and got enough air to prevent mold. Anyway, after about a week in the system all the plants roots turned black, but now they are starting to grow new roots that look healthy. I'm using 2 300W Apollo led full spec lights (2" above plants).

Another root issue that just started in the last 3 days is they are either growing or collecting light brown slime balls. I'll post a picture below. Can anyone tell me what it is and how to get rid off it? And also, since I'm new to this stuff, can someone give me some tips before I do something or don't do something that could harm my plants?

-Jamee