Yeah, I agree it doesn't look like heat stress or root rot. You'd get more drooping.
And with those low levels, burning is unlikely.
You might try a weak solution a complete fertilizer with micronutrients as a foliar feed, it can be absorbed with less dependence upon the condition of the water.
Heave you checked in the runoff tray under that one plant to see if there is any water pooling that doesn't drain? Just a precaution.
Can you see any evidence of mineral deposits on the hydroton pebbles? It would look white, like dried salt.