I agree with Carl, you shouldn't have to replace the sand. I've replaced mine once in 8 years, but that's because I got a new filter and changed over to Zeolite. Otherwise, there's no need to change it out unless you're converting from a Baquacil pool. I think that a 1 1/2 HP pump is definitely overkill for your pool, and probably is making your filter very ineffective. I strongly encourage you to get either a smaller pump or a larger filter.

Janet