I can not believe that everyone here has never made a fish mistake, 'oh I like that one'. Before you know it there is trouble in the little fish tank.
I don't need to go over it again but the tank needs to be bigger for the stocking, the end. Yes more water changes can help as can increasing the filtration but all are short term fixes.
As Peanut said, no need to flame the OP, we have all made mistakes along the way.
Having said all that, I'm a huge fan of growing out monster fish in small tanks, after years of fish keeping you get monsters in two ways:
a. By as a small fish
b. By as a fully grown large fish.
If you go for a, there is no reason that it can't be held in a small tank to grow on for the future.
If you go for b, then there is a fair chance that you have a large enough tank for the job.
It doesn't always work, growing out potentially big fish in small tanks, I've had to move fish on when things that should have financially worked out haven't and I've ended up giving fish away. Along the way the fish were fine, but can lead to a ticking time bomb of stocking.