Hi, Deena,
While I agree entirely with Jason, as he suggests, you may be able to improve your situation dramatically without an entirely new supplemantal system.
I would assume you NEVER run your pump on low speed. With that bather load, you need that pump on high. Secondly, what is your pump run time? My guess is your pretty close to 24/7 but post it up, anyway.
Next, if you can't redo the whole system, you must purchase a VERY, VERY large sand filter. Somewhere in the 150gpm range, IMHO. A triton TR-140 may come pretty close to what you need. Purchasing a filter that is tyhe duplicate of what you have and hooking it up in "parallel" to your existing filter might work as well. In short, you clearly need much more filtration than you currently have and you need to run that pump a lot.
Post back what your regimen for daily maintenance was this Summer and you'll get lot's of ideas on how to improve it. As you can see from the above posts, you must increase your filtration to keep your pool crystal clear next summer.