You know I honestly never have had the problem until this grow and it definitely happened as soon as I switched to bloom nutes.....I kept my plants on grow for the first week of 12/12 at Zandor's and lw's suggestion so I know it wasn't the lighting......definitely the nutes and since the plants have grown like beasts I am starting to see the shift back to the ph rising slowly. I don't think it has anything to do with how old the solution is because it seems to happen even with a fresh res change. I agree with Mr E that small amounts of baking soda seem work better keeping the ph up than the normal ph up seems to.