The next stage of my little project is to turn on the AC when the fish tank gets too warm. I'm not going to try to use the Arduino as a continuously controlling thermostat, just using it to kick it on. Easiest thing is to just put a switch in between the AC unit and wall power. It's an old Fedders through-wall unit. Landlord is a EE, and he says there's no problem switching just one leg of the 220VAC supply. (I had initially thought of using a DPST NO relay.)
So, off I went, looking at SSRs. I'm wondering whether there's any reason to be skeptical about the $10 unit at MPJA, given that anything else I can find is $30 or more (Mouser, Panasonic). The one at MPJA kinda looks like it has a Kodak logo on it, which makes me nostalgic for Kodachrome, but informs me not one whit about whether it's a good part.
BTW, yes, I will be appropriately careful about using mains voltage. I'll be building something resembling this powerswitch tail.