I think you have an algae bloom (green water). Check this thread out from a fish site
http://www.plantgeek.net/forum/viewtopic.php?t=465
You might want to pursue the Magnum filter. I think it would help. You can also use this filter with diatomaceous earth or diatom powder. It provides finer filtration. It is supposed to whip green water and parasites. Here are some more sites.
http://www.aquariumguys.com/aquarium-di ... lters.html
http://www.petsmart.com/product/index.j ... 1&keepsr=0
I am just starting to research the diatom filters, but will probably get the magnum eventually. Like I said I love crystal clean water too. And this seems to be a good option. Chris suggested using an ultra fine filter floss in the canister. Would be a cheaper way to try and tackle the problem to start with. But if the problem is long term you may just want to have a diatom filter ready for occassional but serious clean ups.
On the sand I bought quikcrete medium sand from Lowes. Others here have used playsand. Some fish purists will only use the aquarium sand from fish stores. But the sand at hardware stores is widely available and cheap. I got 50lbs for $6. This is vs the $30-40 for a comparable amount of aquarium sand.
Others probably have more experience and other solutions too. Let us know what happens. Good luck!