It's true. I have and use lake water for my rdwc. I had brown algea and root rot problems for a long time. I researched for ever, tried every sterile solution from sterilizing every thing and using calcium hypoclorite, h202, uc roots to the beneficial route of hydro guard to orca and so on. I mean this went on for I don't know how many runs I did. I kept trying and trying. Starting fresh after cleaning and re sterilizing. To the point I was ready to give up on the rdwc all together. Now I use calcium hypochlorite in my water to sterilize it. Then I run it through a charcoal/carbon filter to de-chlorinate it and remove any more organics. Now that along wasn't enough to make it work. After that I use a brewed beneficial tea with ewc. To inoculate the res water. Now after sterilizing the water and de-chlorinating it and putting in the beneficial tea did i finaly beat the brown algea and root rot. My plants and roots have never been so happy, look so good and growing like I have honestly never seen before. The roots explode in pearly white growth and fill and I mean fill the 5g bucket they are in. The stem on the plants are massive and the plants grow unbelievably health stems and leaves. I am not kidding that beneficial tea makes the plants explode with healthy growth I have never seen before. And I have had no brown algea or root rot ever since as long as I keep up with the tea, but it is more than worth it.
I dont wanna take credit for this though. Do a search on Heisenberg tea and see for yourself.
It honestly worked for me thankfully and sounds like it did for alot of people.