When THC degrades it becomes CBN not CBD. CBN is what gives you the "sleepies" when you smoke pot. Flush, don't flush, people do it both ways. It's certainly not a requirement. Really the feeding requirements slack off pretty drastically toward the end. If I'm growing in soil or coco, I'll usually just use water that last week. In hydro I'm usually just topping off with plain water to keep ECs down.