Hello MontyJ,
I live in the desert also. Lake Havasu city AZ (the Mohave desert). We typically reach 120 Ferinheight by late summer. The last month has been in the 102 to 110 range and it's not even July yet. Growing in the heat has been my greatest challenge. First thing I see is that other than the reservoir, nothing looks insulated. Ya, the panda plastic helps reflect light, but it's not insulation. I would first suggest wrapping bubblwrap (Reflective) insulation tightly around each bucket. Like in these links:
Reflectix 25-ft x 24-in Reflective Insulation
Cool Shield Bubble Rolls and Pallet Covers
I also attached a picture of a pepper plant I have in a similar bucket wrapped with the bubble wrap insulation. Make sure to wrap it tightly. I used velcro to hold it snug. But you can also use a spray (contact cement) adhesive, and probably would be better.
Second thing I see is you have your buckets sitting on a wall. That wall absorbs heat from the sun all day long, and it radiates into your buckets from the bottom. You'll also notice in the picture of my bucket, that I have it sitting on top of a one inch thick piece of Styrofoam. Styrofoam is an insulator and blocks heat coming up through the bottom. The Styrofoam I used is a high density blue-board type, because I already had a bunch of it. But any regular white Styrofoam will do as well. Just make it an inch thick, or two 1/2 inch thick piece to get one inch thickness. Also it may be hard to see in the picture, but I also cut pieces of Styrofoam to place on top of the buckets to block the top of the buckets from the sun's heat as well.
Third thing I see is all those black return lines in direct sun. That is a great way to heat up your water (opposite of what you want). As a test, lay a garden hose with water inside of it out in the sun for an hour or two, then turn it on and hold your hand in the water that comes out. It will be extremely hot, and most garden hoses aren't even black. You get the same effect with the black Polly irrigation tubing. Cover all the tubing to the buckets and back to the reservoir with
pipe insulation. If you need to, wrap the pipe insulation with white duck tape to hold it tightly in place.
Ya, I would recommend a larger reservoir, however they don't make coolers all that big, and they cost a fortune (here anyway). I haven't built one yet, but I have designed what I call a cooling box. It's essentially a costume built ice chest. It's designed to place a container your using for the reservoir in it, while acting like an ice chest to hold in the cold like an ice chest, and you can build it any size and shape you need. Here is a link to the design plans on my website:
nutrient%20Reservoir%20Cooling%20Box.html" target="_blank">Nutrient Reservoir Cooling Box
Also I haven't drawn up designs for it yet, but I have plans to build and test what I call a reverse swamp cooler. If your familiar with swamp coolers, you know the water is nice and cool (mid to low 60's Fahrenheit) after running through the pads (even in high heat). The idea is to use this cool water to cool the nutrient solution. Either by placing the nutrient solution reservoir directly in the reverse swamp cooler reservoir, thus cooling by direct contact with the nutrient reservoir container. Or by pumping water from the reverse swamp cooler through coils placed inside the nutrient solution reservoir, preferably while it's inside the cooling box to insulate the nutrient reservoir from outside heat, and keep the cool in. If you would like to try it, and need help with the concept or designing it, contact me by e-mailing me at the address on my website so I can send you drawings to explain it better.
P.S.
Do you add ice to your reservoir, if not the temp will never go down without some type of cooling, even with good insulation. The insulation just helps hold the cold in longer.