Getting a jack dempsey, how big of a tank do they need?

 
Getting a jack dempsey, how big of a tank do they need?
I've been googling and haven't seen anything on how big of a tank they need. Right now it's in a 20gal the person who owned him before had him in. Not sure that's big enough though.
id say if hes going to be by himself or maybe one other fish you could do away with a 55 gallon for the life of the fish and never have to worry about upgrading again. id search on craigslist for a good deal. you can usually find complete setups on there for around 100 bucks.
The golden rule of fish keeping: one inch of fish per gallon. But going by that logic you could keep a 10 inch Jack in a 10 gallon aquarium...

I would say 55 gallon for a pair if you have good filtration and are a diligent aqua mom...most say 55 for one.