Under gravel filter	Wet-dry filter

    Filters, are one of the most important things you can have when you set up your fish tank.  Above are two different types of biological filters.  The under gravel filter sits inside the tank, underneath the gravel.  It is usually used in conjunction with an air pump (shown below) or a power head (shown below.

Air pump	Power Head

    Both work in much the same way in that they suck water down through the gravel and up through the updraft tubes which causes fish waste and excess food particles to be sucked into the gravel where nitrifying bacteria can break it up (If you haven't read about the tank cycle yet, then you might not know what nitrifying bacteria is).

    The wet dry filter works in much the same way only it is usually housed underneath the tank or on the side of the tank.   Either way, it is housed outside of the tank.  A wet dry filter (shown above) sucks water out of the tank, through the use of a pump, and down into the wet dry unit.  There, the water trickles down over bio balls (the cute little colored balls in the picture above) that house the nitrifying bacteria.  It is then usually pushed through a sponge (or whatever else maybe installed in the wet dry like a protein skimmer or other bio media) and then sent back up the tank through the use of the same pump.

    The choice is up to you which one you want to start out with.  I suggest that the beginner start out with the under gravel filter and as you start to grasp the concept of biological filtration then move on to a wet dry system.

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