Question
I have recently emptied my pond out and filled with new water through a filter for gold fish that live in in (24 last on count).  The pond has plants and a straw bag that has recently been put in.  I have noticed bubbles along the top of the water and when pulled out with a net they can be burst to leave water in the net.  What is causing it?  What can I do to stop it?  Are the fish OK?

Answer
Bubbles are caused by the fact that gas is less dense than liquid.  The gas gets trapped in a layer of liquid.  As the gas rises, attempting to escape into the atmosphere, it remains weighted down by the water.  This forms bubbles.  More than likely, there is a mixture of Oxygen, nitrogen, and a few other elements.  The reason that they are round is because that is the shape with the least surface area.