Hydro. If the plant is overfed, can the ec possibly go down when they are given new nutes ?

Your question is valid. Yes EC can go down when the plants are given right nutes, and the concentration of ions in the solution is almost exactly what the plant need or less. Another case: EC can go down when wrong nutes are mixed together or right nutes mixed wrong way. In such case there’s salt build up. The first case is yours I hope. Plants are clearly having Magnesium and what’s unlikely for hydroponics, Potassium deficiency. When you have K deficiency in hydro that’s the clear sign of too low EC given to plant (of course if NPK ratio is adequate). Because in general Potassium deficiency never occurs in hydro (if growing technology is followed correctly). You need to raise up EC accordingly to the size of the plant and it’s growing stage. Currently they are in flowering which takes the most of energy to produce buds. 350 ppm (0.7 EC) is too low for early flowering. It’s even too low for early veg, imho. You’re giving them incredible mix of nutes from different brands which also may lead into some nute lockout. It’s always better to stick to one brand (grow, bloom, CalMag, ph up/down). They will not grow better when you add so many enhancers and stimulators. In fact to achieve good results, the list of nutrients can be very short: Grow, Bloom, CalMag, pH up/dn, Fulvic acid, and sugars during flowering.