is your fishtank shaded 100%? any insulation?
shade the sides of your growbeds, and shade your gb's in the hottest part of the day
even having a fishtank buried, in heat like that, the growbeds are going to transfer lots of heat to the water.. constant flood may be a bit more "stable" but given a small volume of water, it will still heat up before the end of the day, and will hold the heat longer into the night...
what kind of humidity do you have? (maybe a mister over the gb's)