How do you start a bottom filter for a fish tank?

Answers:
I'm assuming you're asking about an undergravel filter (although corner bottom filters would work much in the same way). You first need to have the plate(s) put on the bottom of the tank. Insert the uptake tubes and cover the bottom with gravel.

Now you can run the filter by one of two ways, either using an air pump or powerhead.

The undergravel probably came with some thin tubing. There should be a rigid (stiff) piece that threads up through the top piece (elbow). Slide this through and you can attach an airstone at the bottom end with a short piece of flexible tubing (having the airstone is optional). Connect the top of the rigid tube to a longer piece of flexible tubing and attach this to the connector of your air pump. If you've got multiple uplift tubes, you can by small plastic or metal connectors to combine the tubing into one or two pieces, depending on the number of connectors on the airpump.

The above will work, but is probably the least efficient method of using an undergravel filter because the flow through the gravel is relatively limited.

Powerheads can be used by sitting them directly on the uptake tubes from the filter plate (you can't use the elbows or rigid plastic tubes with this). The motor driven powerhead moves the water at a faster speed, so more biological filteration can take place. As vort3xyz mentioned, better powerheads can have the direction of flow reversed, so there isn't any buildup of "gunk" underneath the filter which would need to be removed periodically otherwise.

I do use undergravel filters on some of my tanks, but never as the sole type of filtration. I like them mostly for taller tanks, or larger tanks with "messy" fish (cichlids, plecos, goldfish). See more about the pros and cons of this type of filtration in the links below:

---

If your tank is already set up you have to take it apart and put the filter in the bottom of the tank. Save the water and don't clean the gravel because it has all the good bacteria that keeps your tank clean. If your just starting out, put the filter in the bottom of the tank, add your gravel attach the air lines to the air pump and your ready for water.

---

Don't do it... Just get a filter with a bio-wheel from Marineland, penguin, etc... The bottom / ugf filter is 1960's technolgy whose time has passed. Its only around becaue old fogeys will not give it up. Unless you run them in reverse in a sand filled aquarium to eliminated dead zones in the sand, they're not much use compared to altertantive technologies for biological filtration.

The UGF will clog eventually %26 its a pain to service. From day one they start at their peak potential %26 go downhill as time goes by. Bio-wheels are only 3-4 inches wide %26 take little or no service. As they age they work better and at any appropriate size, they're many times more effective than undergravel fitlers.