I used to freak out if the water got warmer than 70f, but the one I've got now the water was usually between 75f and 80f, and the plant didn't seem to mind at all. Couple of sparkly footlongs at least. Water chillers are just too damn expensive.
Did you mix the silica in a solution with other nutes, or add it separately? I don't know much about growing in coco, but for DWC I would add the silica at half the recommended dose as the first ingredient in the mix. By the time I added everything else the PH would balance out (at about 5.8 for DWC)