This is an interesting use for a Unicorn HAT, and shows pretty handily just how things can be used in ways we’d never imagine.
I’m not sure how good Unicorn HAT would be for plants, since it fills out only a portion of the spectrum- red, blue, green, and isn’t really the normal kind of “white” light you’d use to light a tank. I’m not sure whether it would be sufficient to support photosynthesis, but I’d be very interested to hear how it goes.
Sciency stuff that I don’t fully understand aside, we’ve not tested the Unicorn HAT at full white at 80% brightness for any significant duration, and I don’t honestly know how long it’s likely to last. Push it much beyond that and the sheer awesome amount of current the HAT consumes has been known to de-solder the soft-start transistor right off the board. This is why we have a brightness limit in the Python library.
Apart from this case, the heat isn’t as big an issue as you might think, and I’ve definitely run Unicorn HATs hot in enclosed spaces just for giggles. Usually not at full-white, though. If you can observe it running stably for an hour, or two, then the thermal load isn’t going to vary much and it’s likely it’ll run forever.
You could use a few jump wires to separate it from the Pi for better cooling, and possibly even bung on a few heat sinks for good measure ( as you’d likely find on LED lighting solutions ), although I don’t know if the thermal coupling would be sufficient through the PCB resist layer ( the black stuff that sits on top of the copper layer ) to make any difference.
It may be possibly to duty-cycle the 12 hours daytime period slightly, for extra cooling. You could dim the LEDs slightly at random periods to simulate cloudiness and all sorts of things.
It’s been a while since I kept fish, but it’s not unheard of to keep a tank in darkness for large chunks of the day to manage algae bloom, since fish don’t need light ( this may apply to saltwater only though ). Apart from maintaining plant growth, I understand a lot of lighting choices are more for presentation?