That really is a very strong dosage of superthrive and I do NOT recommend it.

So... no shit, you and I are running a fairly similar hydro nute delivery system. Constant stream over hydroton actually puts you in a modified NFT setup. Do you have the tip buried in the hydroton or is there any way that water can splash, pour, or spray onto the lower leaves? I have seen that splotching in my own garden and poor control over fluid delivery was the culprit. I solved it with drip stakes pushed down as far as they would go so that the emitter section was below the lowest leaves. In those baskets, you may have to chop the stakes down to only about 4-5" total length. They're the blue ones, less than 50 cents each from the hydro shop, save ya some troubles.