Hello fellow potheads. I have some issues in my garden today that go beyond my knowledge of growing. One of my girls whom is now 2+ weeks into flowering, just finished reaching it maximum height, but is showing a type of grey white drying on some of the edges of the bottom leaves. I run a dwc system and I normally expect some nute burn with the bottom leaves, but also it appears to occurring at two or three of the middle leaves closets to the fan. Anyway my question was how often should someone running dwc replace the reservoir or rather not replace the water at all, and what my this drying sand like color might be? In vegetative I replaced the water twice, while in flowering only once, and I consistently (everyday) top off my reservoir with slightly ph'ed water, but I highly think that this problem could be related to a build up of salts from not consistently changing the reservoir water, although I've read grow log of growers whom only change the reservoir when switching from veg to flower. My ph is always 5.5 - 6, right now its been in the lower 5 levels. Im using BC nutes and im at half strength, maybe im over fertilizing? Im gonna do some more research on what this might be. The two photos are what 2 or 3 of my lower fan leaves look like, hope someone might be able to analyze my issue. Much appreciation for reading. 
