Question: What is pond scum?

Daphne Ng answered on 10 Mar 2012:

Pond scum is the green stuff that you find in ponds and sometimes fish tanks. It is a general term to describe microalgae. Some are filamentous and form tangled mats while others are single cells and turn the water a pea soup green.

Usually, an excess of pond scum occurs when the water is full of nutrients required for growth of microalage such as nitrates and phosphates. These nutrients come from sources such as fish waste. Microalgae are like plants, except they grow much faster. So with a little sunlight, these pond scum can quickly get out of hand. Too much pond scum is a nuisance as it is unsightly (who will appreciate a view of a green pond or fish tank?) and it has a tendency to clog up the pond, creating problems for cleaning and people who use the pond e.g. swimmers, boaters.

So it is probably a good idea to clean your fish tank regularly (if you have one at home) and not expose it to direct sunlight.