Is that S or Fe deficiency, or something else?

830ppm is pretty low... I think they're a bit hungry and you could up the amount of nutes you're giving them. This is the guidelines I use: PPM readings in soil. Early Growth: 400 to 500 PPM — You won’t see too many particles aside from what’s in your grow medium. Seedling: 500 to 600 PPM — The information above applies here, though any starter nutrients will affect the readings. Early Vegging: 800 to 850 PPM — This is when you typically transplant. At this stage, not a lot of particles are needed. Mid-Stage Vegging: 850 to 900 PPM — You’ll start giving your plants more nutrients at this point. Late-Stage Vegging: 900 to 950 PPM — Nutrient intake increases as your plants prepare to flower. Early Flowering: 950 to 1000 PPM — As your plants grow, they need to eat more. PPM readings will reflect the particles in the increased nutrients. Mid-Stage Flowering: 1000 to 1100 PPM — Your plants’ nutrient intake continues to increase at this stage. Late-Stage Flowering: 1100 to 1150 PPM — This is when your plants are eating the most, especially if you provide additives. End of Flowering/Flushing: 0 to 400 PPM — At this stage, you’ll be flushing your plants. Ideally, you don’t want too many leftover particles at the end of flowering. You also want to make sure the PH of your runoff is within the range of 6.0-6.5 with 6.3 being the absolute sweet spot. Good luck!