Anna,
I'm glad you have been happy with your pump/filter set up. A 1-1/2 hp pump is a BIG pump for a 12,500 gallon pool. Overkill really. My pool is about the same size. I initially had a 1hp pump but it was more than I needed. When I changed it a few years ago, Poconos (our resident pump expert) selected a 1/2 hp inground full-rated pump for my pool and it has been fine. Saves me energy costs and is plenty pump for my pool. I also have a 200# sand filter. I have never had to change anything in my filter. The fact that you are having to change your spider gasket every year makes me wonder if the big 1-1/2 hp pump is damaging your filter. I dunno. I'm not the pump expert around here. Just wondering.

Purplegirl -- with a small 5000 gallon pool, you need a small pump. Anna's suggestion about visiting and talking at various pool places is a good idea except that pool stores are notorious for suggesting people buy pumps that are too big for their pools. Just be aware of this. I don't know my pumps but with that small of a pool, I can't imagine needing more than 1/2 hp. Do they make smaller than that? Your pool won't need too big. This is a case of bigger is NOT better.