kelberi breeding. general q's

Ok just to start off.. I have a male and female kelberi.... I don't have a pair. This is a hypothetical post and nothing more. IF they were to pair off I know they couldn't successfully spawn in my 500 gallon tank with all the other tankmates, but if I had a 125 set off to the side to put them in would it be big enough for them to attempt to do the business? What else would I need? And would it be possible they may not pair off in the 500 but may possibly pair if they were alone in their own tank?
Just to clarify I am hopeful but am not counting on anything. Just want some advice and opinions

Fish are about 15 inches, the 125 is 17 or 18 front to back. Just curious if it may help encourage them to pair if I separate them from all the other fish and keep them in their own tank.