Oil Slicks

for the week of 10/27/99

Some aquarists occasionally notice what appears to be an oily film on the surface of the water in their tanks. This is especially common in heavily planted “natural” aquariums, where surface agitation is purposely minimized to help keep carbon dioxide levels up.

What causes it? Many of the organic compounds created by decaying fish waste and uneaten food tend to accumulate at the air/water interface. In tanks with vigorous surface agitation, these compounds are continually re-mixed with the water, preventing the formation of a “slick.”

Is it a problem? At the very least, it’s an eyesore. The slimy film detracts from the overall appearance of a nice healthy tank. It also interferes with good gas exchange at the water surface, and limits the amount of dissolved oxygen available to fish and plants.

Removal. A temporary fix is to gently pull a paper towel across the water surface, absorbing at least some of the oily waste. Attempting to slurp some up while siphoning during a water change also helps, but not much.

Prevention. The obvious answer is to increase aeration and/or surface agitation by adding airstones or more forceful water flow across the surface. In cases where that is not acceptable, a Surface Skimmer (see image at right) device can be added to the intake of an existing canister or power filter.

“Tip of the week” appeared regularly in 1999 and 2000.