What wrong with my leaves?
 

The nasturtiums are variegated.
Other than that, the Cauliflower leaves, and kale leaves potentially showing chlorosis.
Not sure what caused it but new growth seem much better.
I have some issues with cucumber leaves browning on tips.

Biggest issues is determining appropriate EC or PPM.
This is because my system contains, lettuces (Lower ppm) and tomatoes, cuces, zucchini etc (Higher ppm)

Adjusted today from EC-2.0 (1,000 ppm) to (900 ppm)

Maybe a low nutrient solution content will help with brown tips and chlorosis?

yes, nutrient deficiency is one reason.. other reason may be less of chlorophyll content in your plants because of less exposure to sunlight..
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Hydroponics
 

Being a gardener I can just say that this condition may be all because of the low nutrient deficiency, so look into it you will definitely get the better results.

Thanks! :)