i know this sounds stupid but did u ph ur water after u added nutes and before u put it in the res?
all i can say is change ur res....and if it still does it then it could be ur nutes....or maybe root rot.....or if ur using hydroton and its ur first res,it can make it flucuate
but change it reguardless

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LATAS

but my pots are lightproof, cleaned, temps are 75f and i use hygrozyme. i already changed my reservoir water on both my separate set ups. my plants aren't dying, the roots are great on the 5 weekers, the ones suffering are the 2 weekers, their roots arent growing as fast as they should be

so only the 2 weekers r bad?and there using the same res as the 5 weekers?
could it be that ur over fertilizing them?

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LATAS

wash your hydroton again, if it isn't washed really good it will cause to ph to rise. If your ppm is rising lower your nutes ppm solution. The plant is emptying in the res instead of using them. Good luck

try cleaning the filter in the water pump for debris.

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