Micronutrient deficiencies are avoidable and in the worst case scenario, very treatable. Magnesium deficiency is a common problem. Here’s how to solve it.
Looks like light bleaching more than any deficiency. Low Mg is going to show on lower, older fan leaves first. Being a mobile nutrient the plant will steal it from older leaves to keep the tops healthy.
Raise your lights. LED I presume? Tho HID lights fry top leaves too if too close.
As long as the actual grow tips aren't damaged then the plant will grow as normal. The damaged leaves won't get any better tho but it won't hurt to leave them on and they can still make food for the plants as long as they have green left.
Not sure why you would flush the system tho. Light bleaching is not a nute issue.