Can anyone help me identify the cause of the rust-...

In soil you can grow with a ph between 6 and 7 to be on the sweet spot , in hydro the sweet spot is 5.8 all the run long so your EC is even too high for a classic outdoor grow. Reduce your ph , fix the fluctuation by checking the clay balls ph : did you fluched the clay balls ? because the natural ph of the clay ballas is not good for cannabis , it require a flush to bring the ph to the good level and sometimes it can take up to 3 days so it is a long process but necessary to avoid ph fluctuation. The air humidity is also clearly too low , after the seedling phase , when the plants have 3 or 4 set of leaves like yours , your huidity level should not be lower than 70% , even 60% can be enough but 35% is clearly too low , you can buy an himiidifier or you can spray watter on your plants during night time ( the second solution is not the best ) , or you can even let a water bucket in the tent and let the heat makes some humidity.