Let's not get hasty... It really depends on the size of the pair... Some enthusiasts stress that a successful pair of Dovii require at least 300 gallons of water to be happy... But seriously... that's rather inconvinient for you, don't you think? Absolute minimum for a smaller sized pair, (Let's say 7" male, 6" female) I'd say 120gallons... Keeping in mind that breeding your fish will indeed stunt their growth. It's ideal to have as much space for your fish as possible... but it's not required.
But I guess if you're going to have a huge pair.... or an unproportional pair (huge male, tiny female, vice versa) then 300 gallons might be a better investment. heh..
Chris.