Well, so far I got one inline power thermostat that turns on to cool. Amazon, $19. The sensor is air temp...set the thermostat to turn on (power to aerators) whenever the air temp is above 60...the thermostat stays "on" until air temp falls below 60. Meanwhile, pond temp this morning was 44F...cold water for me! I may drop the setpoint to 55 since the pond is so cold. For summertime, I'll have to get a "heating" thermostat that turns on the aerators whenever the air temp falls below a set temp, say 80F, then to run until air temp gets back up to 80 in the morning hours. These thermostats can be chained so that if one in the chain is "off," there's no power. First in the chain could be pond surface temp, if less than say 82, then "on.." (it will be on all winter and acts to prevent circulating deleterious hot water before it cools in the evening) Second in the chain is air temp, if more than say 60, then "on." (the wintertime setting, always "on" in the summer). Third in the chain is another air temp, if less than say 85, then "on." (the summertime setting, and we rarely get winter air over 85). Modify the connections and these things could be wired in parallel...oh the logic!
At some point a little "pi" computer, long term hourly record storage, various sensors for pond temps (surface and at depth), oxygen at depth, clarity, dissolved solids, and other sensors as smart folk might think to be of value. All recorded hourly or so. Some sensors outputs feed to simple software in the computer to control power outputs. Controlled power outputs could include aerators, surface fountains (cool off the surface?), pumps for circulation, filtering and water falls, powered valves to control pumped flows through bog filters, wetlands, and whatever. The well instrumented pond. I dream of this stuff as just another benefit of having built a small pond of 1/10 acre almost four years ago.