I gave up on the sand bags wrapped in plastic bags. Too much of a mess - the bags always leaked and disentigrated. I use three large oval and rounded river boulders - about 20 to 30 pounds each - that I got a a rock supply place.

They do grow algae, but so did the sand bags. I don't see a difference in the algae problem - there is less of a mess, the rocks don't break open and leak sand all over the place when you take them out at pool closing. For this reason, the rocks are easier to take out then the bags were. I might chnage my mind in the future - I've got a 16 yr old helper now ... :-)

The rocks I got are rounded with no nicks or edges. I space them around the bottom of the steps.