Hey buds, I use an aerogarden aswell, and well let's face it, there is definitely a water temperature issue, even with it all taped off with reflective tape, what I do is I keep all my aerogarden nute tabs frozen in the freezer, when the lights go on and it starts heating up, I stick a couple in there for abit, pull em' out, stick them back in freezer and put some more frozen ones in there.
Sometimes I also use a piece of aquarium tubing, you know that stuff connected to your airstone, and syphon the water out and spit it into a cup or something, then add some fresh cold water