You could always add another airstone to try and increase your DO content, which decreases as temperature increases. This is why high water temps are problematic
I'm sorry, but I need to catch up on my hydroponics jargon, but "DO content"? I don't know what that is.
I think I need a better airstone as the one I'm currently using isn't as efficient as it should be.
I have a question, what grow medium are you using? I use plant-it expanded clay which is the same as hydrotone. Just what my hydro store carries.
I'm using peat moss, we don't have any hydro stores here and hydroton can only be purchased online which I haven't done.