I use 50lb bags of filter sand too, but I use 3. I also drilled holes in the top front of each step.

I put a piece of 40guage vinyl under the steps. Home Depot sells this stuff for putting under shower stalls. The drawback is the rest of the bottom is a tad faded. But it seems to slow algae down slightly. I also hide a floater with a couple of tabs under the steps, set to the lowest level. It SEEMS to help but there's still algae growth under there.