Size:

Size matters! Depending on your aquarium size, you will need an appropriately sized main chamber. The part that matters is the surface area where the filtering bacteria will live. It can be expressed as volume of Bio-Balls (or other medium used). This page describes a filter that uses approximately 1.5 gallons of Bio-Balls, sufficient to serve my 50 gallon freshwater tank with goldfish. Goldfish are known to produce a lot of waste nitrogen. This is mainly due to their weight. They produce about as much as any other fish per pound of weight, but they do grow big, and as a result nitrogen load on the tank becomes significantly bigger then in many other cases. Here is my guess as to how big of a filter you might need along with pumping requirements:

I do not believe that a Wet-Dry filter is the best choice for a small aquarium. You can achieve similar effect on a small scale by using a thick layer of sand and small gravel on the bottom. I have had a 10 gallon aquarium for years with close to no filtration, and no detectable nitrogen (all three forms). It has about 1 inch of fine sand on the bottom covered by 1/4 to 1/2 inch layer of small gravel. Heavily planted, of cause, to remove nitrates produced by bacteria from ammonia and nitrites.

Materials needed:

Qty. 2 two-gallon buckets. Hardware stores are the best source for these.

5 gallon plastic container. The 2-gallon bucket should fit inside.

A piece of filter material for pre-filter that will cover the bottom of a 2 gallon bucket.

Submersible water pump. The pump size depends on your aquarium size, see table above. The rule of thumb is that your pump should be able to turn 2-3 times the aquarium volume through the filter in one hour.

Plastic hose to go from the pump to the aquarium and a clamp to hold the hose on the pump outlet. You can use a large cable tie instead of the clamp, but make sure you tighten it well.

To make life easier I recommend making a filter outlet that sits in the aquarium from PVC or CPVC pipe. For that you will need about a foot of 1/2" pipe and three 90 degree angles.

Making the drip plate:

The drip plate is the device that distributes water evenly over the surface of the Bio-Balls. I made mine by cutting a bottom along with 2" of the side off  one 2 gallon bucket. It fits ideally inside the other bucket, is cheap and easy to come by. I also combined my drip plate with a pre-filter by cutting a circle from filter material to fit perfectly in the drip plate. I drilled number of holes through the cut-off bottom using 3/32

" drill bit. You can also use a small nail - if you hold it with a pair of pliers and heat it up it makes a perfect hole every time. Do not make too many holes: you can always add more if needed, but taking them out involves buying another bucket, cutting the bottom off and drilling fewer holes the second time around.

Bio-Ball container:

Take another bucket, remove the handle and drill number of holes in the bottom using a large drill bit (I used 1/4 inch bit). I also drilled a row of "safety overflow holes" about 2" from the top, but this is probably optional.

Fill this bucket with Bio-Balls leaving about 2" on top for the drip plate. Make sure the Bio-Balls are evenly distributed.

Now you can fit the Bio-Ball container inside your main chamber container, and place the drip plate on top of Bio-Balls:

The pump goes on the bottom of the main container next to the Bio-Ball bucket. In the picture above the mounting plate of the pump is attached to the right wall of the container with three suction cups.

Water outlet:

To make my life easier I used a small piece of 1/2" CPVC (this diameter was chosen because it fits nicely inside the hose coming from the pump) and three 90 degree fittings. The picture should be self-explanatory. The metal clamp is to prevent the hose from sliding off the pipe and flooding my living room. The last 90 degree fitting (the one submerged in the water) is not glued to the assembly, just fitted on tightly. This way I can adjust the angle of the output stream. You should also make a small 5/32", or so, hole on top if the top left angle fitting. This is a safety feature: if the pump brakes or loses power, the hole will prevent water from being sucked back down from the aquarium through the outlet and through the pump into the filter. I have had an overflow in my bottom chamber as a result of such accident when I did not have such a hole.  This hole will also add some aeration to your aquarium by making use of a so-called venturi effect. Basically the water flowing through the pipe will suck air in and carry the bubbles down into the aquarium.

If you have any questions or suggestions for me, I will be happy to hear from you. Email me: denis at gerasimov dot net. Put Wet-dry in the subject please.