jus got one on friday for my 180G tank and it is pretty damn amazing!
had to have a re shuffle of some rocks so that i can see behind them all otherwise i wouldnt of seen him yet but im hoping hell come around and get braver!
when i used to have 2 they were my favourite fish! they got to about 6inches well under their max size but i gave them away after my angels didn't like them this was years back now but i've now learnt they needed a bigger tank then i could offer them anyway.
I used to keep mine with discus but then i moved it over to my larger tank. I think that they are really gr8!!! They would be fine with cardinals when they are very small, but when the BGK gets big enough they will go.
80 gallons is too small IMO. I saw one on a Amazon television special a few weeks ago that was nearing 3 feet in length. They need at least I would say a 100 gallon tank.