Cant let the nutrients sit aorund like that, they will go bad. If they contain organics the organics will rot and the smell is from bacteria.....the BAD kind!
Only mix enough to use for EACH watering.
Some products are best kept in the fridge to prolong there potency and shelf life. Some products have preservatives added and you do not have to worry about it too much. But preservatives are usually a no-no in organic formulas....sooo its like a catch 22 or something.......