seriously find someone on here with a large tank close to your location...for the good of the fish. Otherwise it will die a painful death in a very short while
Take it back to petland and get a Raphael cat they can live in your 55. The only way a RTC will not outgrow a 55 is if you kill it before it gets a chance too.
Next time do some research before you get a fish even if it's a free fish.