IMO, no. This might not be the most popular opinion, but I don't think a 125 is even big enough. I like to keep a fish in a tank (very rough guide) that is around 5-6x the length and 2x the width of the fish's length. So a fish that's 15" should be in approximately a 72x30" footprint at least (if you've got the money go 96x30"). I'd say at the very least a 180 gallon for an adult pair.
how many 15" festae out there?? and its gonna take years and years to get that big. and with that logic you could be ok with a 29 gallon till theyre 6" which could be a year with how slow festae grow.