Nute burn???

would recommend changing the reservoir solution one every 4 to 5 days and see if it helps. in hydroponics ; it so happens that the ppm or ec count remains the same but the plant has actually used up some nutrients and they even exchange back some in water ; creating deficiency in one and excess in other ; also many hydroponic growers do not suggest topping more than twice a week ; as they altogether change the water or keep a circulating system of solution. so changing whole reservoir water should definitely help you in this case.