Salty food

anywhere fertilized water evaporates repeatedly, something can be deposited -- organic, salt, (irrelevant terms for this context) whatever... a solute is a solute... when the moisture is gone it's a solid again and a risk for 'buildup' in a solid medium or more concnetrated fertilzer in the hydro setup than you intended etc...

When you buy hydroton, wash it a half dozen times. When you swap plants/harvest, you wash your hydroton until clean. When you put a plant in, cover the hydroton.

Yeah bro, that’s salt buildup from the nutrients. It happens when the top layer of clay pebbles dries out and the salts crystallize there. It won’t kill the roots directly, but it can mess with pH and cause nutrient lockout over time. Just rinse the top pebbles with clean RO water once in a while and make sure your drip lines don’t stay running 24/7 give the medium some time to dry a bit between cycles. You can also lower EC slightly for a few feeds to balance things out

Yes, those are most likely crystallized nutrient salts. When the nutrient solution is drawn up from the reservoir to the surface of the clay pebbles and evaporates there, the dissolved minerals (salts) are left behind. The symptoms on the leaves (chlorosis along the edges, rust spots on the tips and margins) indicate a problem with the uptake of mobile macro- and micronutrients. This could be caused by an incorrect pH level, which is the most common issue in hydroponic systems. An improper pH leads to nutrient lockout — meaning the plant can’t absorb the nutrients even though they’re present in the water. Optimal pH for hydro: 5.5 to 6.5.