I am a new marine aquarist. I just added live sand and rock to my aquarium. There seems to be a thin oil slick on top of the water. Is this normal during the cycling period? Should I be doing water changes while the tank is cycling?
It is not unusual to see an oily film on the surface of the water during this period. It is caused by the hydrophobic properties of dissolved organics in your water. The dissolved organics will form on the surface as they are repelled by the water in the tank.
A protein skimmer use these properties to work. By mixing tons of tiny air bubbles with the saltwater, the dissolved organics are attracted to the bubbles and move toward the surface to get away from the water. Eventually they build up enough to overflow into the collection cup of the skimmer. Here is an article on how skimmers work for more information.
If you don't currently have a protein skimmer, I suggest adding one to the tank. It will help get rid of dissolved organics in your aquarium. Sometimes adding a skimmer isn't enough though. You might also need to break the surface tension to get rid of the oily film. This can be accomplished in a couple of different ways.
One way is to use a powerhead to ripple the surface of the water. This will help break the surface tension. Another way is to use a surface skimmer on the tank. They hook up to certain filter systems to help break surface tension. Some hang on back (HOB) protein skimmers, like AquaC Remoras and CRP BakPaks, have an add-on pre-filter that will skim from the surface to help remove the oily film you mentioned.
As far as water changes go, yes you can do water changes at any time during the cycling process. They will be beneficial to the tank.
If you have any additional questions, concerns or comments, don't hesitate to let us know.
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