What do I do about this fungal infection?

Prbably caused by you having the humidifier pointed directly at the plants. If the xylem is completely rotted, the plant will die.........the dying leaves may point to this being the case already. Tie some strings/twine from the ceiling down to the plant to support it and stop it leaning..........if you are lucky, it may pull through.........but you will need to lower the humidity big time while it recovers......as in, turning off the humidifier. I would not treat it with anything, keeping it dry is essential.........it is now a question of how much this auto wants to survive. If you still have spare seedlings, replanting might be the best option. BTW, seedlings do not need to be in clone boxes/humidity domes...........it can cause stem rot........

She is a goner unfortunately, you can support her up but it's not going to help in the long run. I would start a new seed..

Hey there… I’m sorry to say it but it’s time to call this one over…. Don’t waste your time trying to baby it back to life. If by some change the plant doesn’t die it will never be a good plant. Half the main stem is already gone it will always be thin like that but 99% it dies b4 harvest. Just start some new seedlings and make sure to have god air flow and control humidity and temps when doing hydro systems. I’d suggest trying soil it’s 100000x more forgiving.

looks like it survived dampening off. will likely need to support it a bit. I don't know how the fungal diereses processes or if it stays in the tissue so clones maybe out of the question. Your going to need to do some reading.

Das sieht schwer nach Pythium aus. Wie sehen die Wurzeln aus? Wie hoch ist die Wassertemperatur? Wie viel Luft pumpst du in den Behälter und wie groß ist der? Was für Dünger nutzt du?

If the infection is fungal, the key is to control humidity and root conditions. Carefully remove any affected tissue and make sure the roots are clean and well-oxygenated. Crop Defender 3 can help reduce fungal pressure when applied to leaves and the affected area, but it may not eliminate an internal infection completely. Ensure good airflow, keep water temperature between 18–21 °C, and avoid nutrient buildup. If the stem is weak, support the plant with a stake or tie to prevent it from leaning. Recovery can be slow, but if healthy tissue is developing and the roots stay white, the plant can gradually recover.

Wow that does look dodgy with the stem. Possibly was getting to wet. Once weight starts packing then I think that will just snap from weight. I know your in dwc so my suggestion may not work but I'd put a stake into the clay pebbles and wedge it into one of the holes in the net pot then tie the plant to the stake at various heights to support it. I had a plant look like that once in dwc and it was a plant I had the water level way to high up the bucket.