Eh, wait a few months. Of course then we have like 95 degree days and you'd have some poached fish without proper cooling and that might not be an easy project outdoors.
I'd guess not the best choice in any situation. San Diego area might be a good place to do this. Avg temp is like 72 all year round and somewhat humid.
I am contemplating insulating my garage, covering the walls in MDF wood sheets for added insulation, painting the MDF gloss white, and loading the garage with tanks, but that's a good amount of time away. Even that will likely lead to excess heat over the summer so then I'll need A/C units or swamp coolers, or some industrial fans to generate tank sweat (evaporation).
If you wanted an outdoor tank, you could keep MANY tanks and them bunched together to conserve heat/cold loss. A single 40g or even 75g would be too susceptible to temp variations, I think.