Why is top column of protein skimmer foam blue?

The blue foam at the top of a protein skimmer is typically caused by the presence of dissolved metals, particularly copper. Copper can react with oxygen in the air to form copper oxide, which is a blue-green compound. This compound can then be trapped in the foam produced by the protein skimmer, resulting in the blue coloration.

In some cases, the blue foam can also be caused by the presence of certain types of organic matter, such as lipids or proteins. These compounds can also react with oxygen to form blue-colored compounds. However, in most cases, the blue foam is caused by copper.

The presence of copper in the aquarium water can come from a variety of sources, such as metal fixtures, copper-based medications, or even the tap water itself. If the copper levels in the water are too high, it can be harmful to the fish and corals in the aquarium. Therefore, it is important to monitor the copper levels and take steps to reduce them if necessary.