I've thought about a few home automation applications in the past, but I'm not ready for a controller yet so I haven't given it more than passing thought, and I'm not sure yet if it is something I'd get real use of or not.
1) local alarms - like turn a living room light a particular color for various parameters out of spec - so the alarms have different urgency
2) thermostat integration - where I live, it's hot most of the year. The furnace runs sparingly, and I might swap back and forth between furnace, AC, or nothing in the winter. If I was controlling the temperature of the tank directly, I might change the deadband of the heater depending on the house climate control mode - maybe fewer cycles is less risk of heater failure. Conversely, if a heater failed and I shut it off, I might alter the house temperature to give me longer to address the problem.
That said, the hubitat intrigues me, so if I got one, I'd probably want to link in my aquarium somehow just to demonstrate I could.
I'm a year or so from a controller in my mind, but this looks pretty interesting.
I like the idea of using a light in another room to alert you to an issue with the tank. My aquarium is in the entry of my home, but I spend most of my time in the office in my bedroom. A visual cue that something is amiss could really get my attention when I might be otherwise super plugged into what I'm doing on the computer at any particular moment.