Base of stem dies and breaks suddenly

I am trying a little experiment with two autoflowers. Both have been in dwc in separate res. One has been getting the normal h2o2 and the other an beneficial bacteria formula. From the start the one getting the bacs was bigger and bushier. But a few days ago it started having a few leaves yellow then not looking so good overall. Just now I was pruning a few dead leaves (these plants are now 5 weeks old) and the damn plant just broke off at the base with no pressure on it. There was a black line around the base of the stem and it just came apart there.

This makes me very hesitant to deviate from the sterile h2o2 res I have always used.

the black line is the algae and it destroyed the cambium layer, cover the pots and keep the light off the rocks

Sounds like stem rot.

I have had that too recently and baffled me, the answer given makes sense since I been starting a Nitrozime Algea foliar spray in veg and couple plants did the same thing you stated just broke off at base for no reason when the whole plant looks health as @!)&@)*&!.

I was able to clone them and save them but now I am making sure the Nitrozyme is not sprayed near base and they are covered.

Thanks.

I'm going back to h2o2 and a sterile res. Thanks all!