You may be able to house a BGK in your 80g for life if you wanted. I raised mine from 1.5-4.5" in my 29g, from 4.5-6.5" in my 40Breeder, and he is now in my 55g thriving at 6.5". They are very slow growers, usually growing around 6" per year.
They are pretty docile for the most part, but once they establish a "house", some will defend it by doing what I have coined "the BGK headbutt"... It's not really harmful to the other fish, but it gets the point across. I kept my BGK with my 3.5" sev in my 40 breeder for months with no issues. Other fish seem to be confused by the BGK. I had a pair of sevs that would keep a distance from the BGK and just sort of "watch it".
I know others will tell you that they will need 100+ gallons to live for life, but after owning one for a while (a year), and witnessing their behavior daily, I can tell you they don't need as much space as most will lead you to believe.
Just my opinion, nothing more.