Pre-Filters - The  piece of equipment used to skim water from the surface of the tank.  It consists of two boxes, usually made of acrylic.  One box hangs on the back of the tank, while the other, smaller box is inside the tank.   The two boxes are connected by a siphon tube.  Water flows over the top of the smaller box, and is sucked into the siphon tube.  Water then is collected in the outer box, where it usually passes through a round sponge, or pre-filter, removing any large waste particles, and preventing any critters from entering the filter system.   The water then pours down a tube which leads to the sump system below.

Below is a diagram of a Pre-filter box that I found on the web.  It is a LifeReef Systems product, judging by the logo!  Of course, the purpose of the drawing is to illustrate what a Pre-filter is.   I own a UltraLife Reef Products Pre-filter box, as well as a DIY model I fabricated at a friend's plastic shop.

I have made a few easy modifications to my pre-filters.  First, I lost a beautiful Orbic Bat in my siphon.  He wandered a bit too close to the box, or just jumped in - I'm not sure, I obviously wasn't there!  As big as he was, the only position he could be in was laying on his side - and part of his body blocked the siphon, sucking him to death!  Not only did I lose a fish, but I had about 7-8 gallons of saltwater from my sump on my living room floor!

To remedy this, I installed some black netting around the outside of the inside box, and made sure that it extended all the way up to the glass canopy.  This will prevent a fish from getting inside.  Of course, it also gives algae another place to grow!   I remove it about once every other month to give it a thorough cleaning.  I also installed a float switch that will kill power to the pump if the water level reaches a certain height, preventing the floods!

The second modification I did was to get rid of the annoying "gurgling" sound the box makes.  This was achieved with a piece of airline tubing, about 18" long.  I taped the top into a loop about 1 inch in diameter to make a hanger, and placed the tubing  down the PVC pipe in the outer box.  You may need to adjust it up or down until you find the air pocket that's causing all the noise.  Once you find it, tape the tubing in place!  I have read that a partially closed ball valve inline on the pipe will also prevent the gurgling.  I have several ball valves for redirecting water, but the airline tube trick worked for me.  If it ain't broke... don't fix it!