How to Build an In-Tank Overflow

Draw out your design to scale to help prevent mistakes as you build. The basic construction is two connected boxes where one hangs in the inside of the aquarium and the other hangs on the outside. The box on the outside should be at least 1 inch lower than the box on the inside. The bulkhead will go into a drilled hole on the bottom of the outside box. The siphon tube will connect the two boxes.

Cut a piece of acrylic long enough for the inside walls of both boxes plus about 2 inches. This sheet will form the inverted "U" that will hold the overflow boxes onto the aquarium.

Heat the acrylic sheet with a heat gun and bend it so that one side is about 1 inch lower than the other. The longer end will be the box on the outside of the aquarium. Keep the heat gun moving along the edge of the plastic so it doesn't melt one area. Let the acrylic cool down and harden before continuing.

Cut sheets of acrylic with a saw to form the sides and bottom of each of the two boxes. Sand the cut edges with fine sandpaper to get the best adhesion when you glue.

Drill a hole in the sheet for the bottom of the outside box large enough to fit the bulkhead. This hole is where the water will drain out into the sump. Center the hole on the sheet so you have enough clearance for the edges of the bulkhead. The sheet with the hole and bulkhead will be on the bottom of the box outside of the aquarium.

Fit the bulkhead into the hole so that the gasket is on the inside of the box. This is easier to do before you assemble everything.

Cut slits in the top of the sheet that will be the front of the inside box using a saw. Make slits on the top of the front side of the box so that the water passes through the slits from the aquarium and flows into the box. These slits will allow water to pass through them but will filter out large objects, such as dead fish. Make the slits about 1/4 inch wide and spaced about a 1/2 inch.

Glue all the sheets in place to form the boxes using the acrylic glue. Acrylic glue will weld the two sides of plastic together forming a very tight bond. The glue then evaporates away. Let the glue fully dry before proceeding.

Seal all the seams of both boxes with aquarium-safe silicone. The acrylic glue should have formed a water-tight seal, but a bead of silicone will make sure any small holes are plugged up. Let the silicone fully cure.

Screw in the PVC pipe into the bulkhead on the inside of the box. Cut the pipe so that it is a few inches higher than the bottom of the box. This will allow water to build up in the box before it flows out and will keep the siphon tube full of water if the power shuts off. Without this when the power turns back on, the overflow will not work and the return pump from the sump will overflow the aquarium.

Hang the box over the edge of the aquarium. Attach your hoses going from the box to the sump. Fill the inside box with water.

Insert the siphon tube so that the end in the outside box is about 1 inch lower than the end on the inside box. This will keep a siphon going. There are a couple of ways to start the siphon. One is to string an airline tube into the siphon tube and suck out the air. Another is to fill the siphon with water and quickly invert it into the overflow box.

Check for leaks and make any necessary adjustments once the siphon has started. The overflow box should run on its own from this point.