I've seen many newbies changing from FW to SW tanks and seen many mistakes being made so i figured i will write a comment on the importance of surface skimming in SW tanks which most FW tanks don't have. This issue is usually overlooked by most newbies getting into SW tanks.
What is the slick and how does it get there?
Remember that SW is heavier than FW, hence the fact that we measure 1.026 SG for SW (Guess what FW measures, yep ONE). This creates what i call oil on water effect. If you ever had a SW tank without surface skimming you'll notice an oil slick film, always on top of the surface of the water. To my knowledge the only way to remove this slick is by either manually removing it or by surface skimming.
What does this oil slick do to the tank?
This oil slick is actually protein waste floating on top of the water. It comes from fish, coral, food, or other waste floating to the top of the water. If not removed it will accumilate there and cause your light to be less efficient (important for corals), and will break down and cause phosphate build up, which in turn causes other problems, and it will cause poor oxygen and nitrogen exchange between air and water. Those are just a few but most important issues it can cause.
So what is surface skimming?
surface skimming is exactly what the title says. By using an overflow box, the surface of the water is being skimmed by constantly adding water from a sump to the tank and the water then overflows into a box which then goes to the rest of the filters. What this does is the surface of the water is being pulled into the box and forced to be filtered. This simple procedure will leave your water surface crisp and clean and will make a more healthy aquarium will less issues to deal with.
What does an overflow box look like?
Well today there are many products that surface skims. From buy over the counter to actually incorporating a built in design over flow. Here's a picture of a buy over the counter product that does surface skimming.
here's a built in skim box from my aquarium.
A pic of the back of my built in skim box showing how the water gets to my sump.
and here's my overflow box in action. Notice no film on the surface. The surface of the water is crisp and clean and void of "oil slick".
And here is another aquarium that is designed for SW that has incorporated an overflow box
and here's a pic of a nano type aquarium with a built in design
There is also another product option that surface skim without a sump being required (hang on back type filters) however the disadvantage is that these products require the water height in the display tank to be relatively stable. This means that you will have to add top off water on a daily basis and sometimes more than once a day depending on tank size, how dry your air is around the tank, the amount of air exchange at the top of you tank. This is a committment you should consider before using one of these hang on back type filters.
Hope this helps newbies in designing and understanding their tanks and the importance of surface skimming. So, don't just run out and buy any aquarium without thinking of the overflow options you'd use for surface skimming as this is an important part of filtering a SW aquarium.