🌱 I am certainly not an expert on getting rid of root rot, but I am an expert at
getting it lol. I have done 5 successful hydro grows, with good yields, but I always seem to get root rot at least a little, and pretty significantly when I grow in the summer. Luckily, I feel like once you get past the first month of the plant's life, it becomes a lot more resistant to root rot.
My plants from my last hydro harvest. This was a little over a year ago so I'm likely a little rusty (I've been growing in coco and soil since then to try to get out of my comfort zone, though I do miss hydro!) but I harvested a little over 16oz (a pound) off these two plants (left is Sterling Haze by Nirvana, right is Haze 1 by Nirvana) under a 600W HPS in a 4x4 tent.
They were dying of root rot in the beginning, and look at them at harvest! It can be overcome!
Haze 1 by Nirvana
One thing I've found that's helpful for me when I have a young plant with root rot is to stop moving it or do anything that disturbs the roots. If I have to look into the reservoir or adjust the pH, I use an alternate net pot and don't move the lid at all. I put reservoir changes on hold and just top off with nutrient water as needed until the roots are growing lots of new white, solid-looking strands every day! The only thing I "mess with" is the pH. Then I go back to normal once the plant is growing well again.
I never tried SM-90 for root rot because I just heard of it a few months ago, but I did use it on my soil plants outside that were suffering from fungus gnats, and they do seem pretty happy since then. It seems to have helped some hydro growers with root rot, and I would be really curious to see if it helps more growers with root rot! But I've also learned that what works for some people doesn't work for all people, because different organisms can cause root rot in different parts of the world, and in different setups, so you have to work with your environment to see what works best for you.
What is the temperature in your grow space? Is there any type of breeze or circulation plus lots of fresh air? The air pump needs access to cool, fresh air full of oxygen. You don't want it blowing hot, stale air into the reservoir. If it's really hot in the grow space, sometimes you can place the air pump outside the tent, so it's pumping fresh air from the room into the reservoir, instead of tent air.
Is it possible to see a picture of the seedling in the setup under regular lighting? Did anything different happen right before you saw the root rot get worse again? Any difference between the plants that have and don't have root rot? Is the water at least an inch below the bottom of the net pot (so there's an air gap at the top of the reservoir)? I've heard this helps prevent the main stem from rotting, and also helps get a little more oxygen to the roots. The splashing from the reservoir will make sure it always stays misty up there so roots never dry out. 🌱