Question
The rocks in our man-made pond (no fish) have become covered with green slime and release air bubbles from underneath when they are lifted. There is no evidence of algae growth on the water surface. We have treated with aquashade, cutrine and sludge remover bacteria blocks this season.  Possible over or under-use of chemicals?

Answer
There is a simple solution as long as you don't have any fish, snails, frogs, reptiles, cats, birds, or plants in your pond.  You can give the pond a thorough cleaning, refill with water, and add bleach once a week.  If you do this, you can use a swimming pool test kit.

The more natural approach is to add plenty of plants to the pond.  The plants use up the nutrients that the algae would use otherwise.  I still recommend the thorough cleaning.