welll a few things.. what aro is it.. wat are you feeding it.. how big is its tanks... are there conflicts in the tank. theres more but sum of our more hardcore aro fanatics can help
You're averaging an inch and a half a month ... you have nothing to complain about... just relax and enjoy it's time as a little guy because it will only last so long... aros have a relatively long life span... it's not like they grow from a fry to a 4ft adult overnight....