I would say that even now, it would be affecting the fishs' growth and development, so it can't be kept in a 50 litre aquarium. And what are you doing about the fact that it is a con-specifically social fish? Two months in solitary confinement is bound to screw with anyone.
And even when you get a 250 litre one, it will still not be big enough to last the fish a whole lifetime.. when will you upgrade again? In even 6-8 months, it will most likely be affected by the volume and length of an average 250 litre aquarium.
If you are determined to keep it, and I recommend that you do not, your best bet for making sure it grows properly for the next two months is to do 50% water changes every or every other day.
I'm not sure you realise how big this fish can potentially grow. Get out a ruler and measure out 16 inches (40 cm). If you provide a big enough aquarium and good enough conditions, that is how big *each* of these fish can grow. Then add to it that they should be kept in groups of 6+, but do much better in groups of 10-20+, live for 15-25+ years and think again about how you will look after it in the long term.