My roommates and I have a fish.
We’ve managed to keep him alive and more or less healthy for the past two years, but one thing we’re not super reliable about is turning on and off the tank light, which (I’m told) is important for fish circadian rhythms or something. (I’m not a biologist; ask the roommates!)
To solve this, I used an ESP8266 to automatically turn on the lights in the morning and turn them off in the evening. Because the ESP doesn’t have a built-in RTC, I sync every ten minutes with the NIST NTP servers (which likely makes this the most accurate timekeeping device in the house other than cell phones!) and fade the lights off at 7am and off at 9pm.