I purchased a used CropKing NFT system and have tried to use it 3 times now and each time the roots rot or burn off.

I've replumbed the system entirely. I cleaned the system thoroughly and ran a bleach / water solution through it for 24 hours prior to my last addition of fresh lettuce. I even tried Rockwool and a synthetic grow plug and the same results happened on both.

I use the same water and nutrients in my bato bucket system with no troubles.

Is it possible that my reservoir has some sort of contaminant in it? I scrubbed the heck out of it with a bleach solution.

Do you have any ideas? Oh, I grow beautiful lettuce and bok Choi in 5 gallon buckets with the same water and nutrients.