Why? You'd have to have a HUGE one, with much larger than a 20w light to make it worth the time. Then you're costing more money, more maintenance. Water is cheap, maybe you'd be better off rigging up a better way of changing water?
The gauges on the side of heaters are useless unless they can be calibrated. Ignore them for anything but trying to get it into roughly the right range..
Tear that off and throw it in the trash, they're garbage. The internal one you have is good. Use that.
Probably $5000, requires installation in your cannister filter by a certified aviation mechanic at $200/hour then inspections after every 100 hours of usage and annually. Also has a 2000 hour time between replacement.
That's pretty much the process for every cannister. I've never run into one with a "Priming button" or pump or whatever that actually worked. But the above process, with maybe a few shakes to clear bubbles out of the pump works every time.
Varies hugely by individual, and what you mean by "Best Personality",but in my experience it tends to be the bigger SA cichlids. Best I've had was severums, had the most individuality and behavior.