Component Listing, (measurements are approximate):

This type of filter will handle a very large fish population and is overkill for maintaining, for example, 3-4 Oscars in a 180.  I typically keep 20 to 30 large fish in my 180 gallon aquarium.  Although not a true wet-dry in the strictest sense of the definition, this filter by my measurement is 75% to 90% as effective as the same bio mass in a wet-dry.  There are no dead spots, (bio media areas rarely in contact with water), in the bio chamber.  If the water flow is turned off the live bacteria will survive in my flooded bio chamber many hours before starting to die. The construction materials for one of these DIY 40 gallon units will cost about $150 - $200.  A similar wet-dry filter found in pet shops retails for $275 - $300.  These have about one cubic foot of bio media and a 25 gallon sump as does mine.

 Fiber filter drawer:
    15" wide by 20" deep by 9" high, cost $10 - $15, available at Target, Kroger, Lowes

Fiber filter material, 2-3 layers:
    Top-course, Center-medium, Bottom-medium or fine, final bottom layer positioned over the holes must be coarse allowing water to pass freely.  Fiber filter material is available at pet stores and pond supply dealers.  Have the material cut to the width of your fiber drawer.
Use caution when purchasing fiber from other than a live fish dealer.  Some of the Home Depot type filters are for furnaces and often are treated with chemicals to attract and hold airborne dust. If the material, usually blue in color, looks good and has no oil treatment I will purchase a pad, wash it thoroughly then test it in a 5 gallon bucket with 2 or more live feeder goldfish.  If they live a week and there is no sheen on the water's surface I will put it into service.

Bio filter tub:
    The tub should be translucent so that water levels can be observed.  Measure L=up to 30", W=23" maximum, H=14"-18".  The total vertical clearance inside your under-the-tank wood cabinet should be 2"-3" more than the height of the bio tub together with the fiber drawer assembly.  For example the bio tub might be 16" with the lid on.  The fiber drawer might be 9".  The total with the drawer assembly resting on top of the bio tub might add up to 25".  So, you should have at least 27" of inside clearance.

Second filter chamber, for pumps and heaters:
    This tub should be translucent as well if possible.  If you have a 120 gallon which is 48" long then the two tubs when combined can't exceed 46". This measurement includes the separation between the two tubs allowing for the water flow connection.  Measure L=up to 30", W=23" maximum, H=14"-18".  Note the height of the second tub should match the first tub.

The connection between the tubs should be at least 1-1/2":
    I use 1-1/2" bulkhead fittings available at some pond supply dealers and pet shops or Aquatic Eco Systems on line, cost $10 each.  You must use some type of quick disconnect such as a compression pipe fitting or a rubber sewer pipe connection with hose clamps, cost $5.00.  A coarse strainer must be positioned in the bio chamber to prevent the media from entering the second chamber.  An 18" length of 1-1/2" pipe drilled with holes and a cap on one end will work as a strainer.

The overflow box is made from scrap plastic which is cut to size and glued together:
    The box assembly can be fun and is quite easy.  If you are not able to cut the panels to their exact sizes most plastic shops selling scrap Lucite or Plexiglas by the pound will do it for a small cutting charge.  The box will hang on the back or the end of the aquarium and will draw water from the tank via 2 to 4 siphon tubes.  The inside dimensions of the overflow box can be:  8"-12” wide by 4" front to back by 10"-12" tall.  I use 1-1/2" bulkhead fitting in the bottom of the overflow box to achieve maximum water flow.  Depending on the height of the box, slip or thread an 8"-11" piece of 1-1/2" diameter pipe into the bulkhead fitting to use as a standpipe which regulates the height of the water in the tank.

    The 1-1/2" hose can be flexible plastic hose or Home Depot ribbed trash pump discharge hose.  With this setup I can achieve a water flow of about 1,200 gallons per hour in my 180 gallon aquarium, using 3 small Laguna Power Jet Pond Pumps, Model 3000.