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I have an external overflow and it is not syphoning. The tank is filled. Sticking up out of each drain hole there is an air tube and there is a small peice on top of the overflow that looks like it connects to an air tube. I also have a small green tube with a checkvalve on it that I dont know what to do with. Please help! how do i get it to syphon?

502 Posts
On my overflow, I have a rainbow tube sticking from the back of the overflow, into the part that is actually in the tank. I fill both boxes, the one in the tank and the one hanging off the back, and then I run a small piece of airline tubing in to the rainbow tube. Suck on the airline tube until water meets in the middle over the tank lip. Take the airline tube out of the rainbow tube quick, or air will get in there. Then I am done. It sounds like you are supposed to hook a piece of airline tube to the top of the overflow, fill both sides with water, and start sucking,
, hehe, when the water is drawn up from each side, and meets, take the hose off, or use the check valve to close it off, hope this helps!

3,133 Posts
CS102 Continuous Siphon Overflow


CPR's Continuous Siphon Overflows provide the most efficient means of getting water to an external filter without expensive modifications to the tank. These precision-engineered overflows skim water from the surface of your aquarium at flow rates up to six times greater than units with tubes. Advantages such as water level adjustment, quiet operation, fish screen, and a black top to reduce algae growth all make the CPR Continuous Siphon Overflow the best on the market.

To place your overflow into operation:

1. Open the packaging carefully and inspect the unit for damaged or missing parts.

You should have:

(1) Overflow body
(1) Fish screen
(2) Pre filter screens and sponges
(2) Vent tubes with small "O" rings
(2) Bulkhead assemblies
(4) Adjustment screws
(1) Airline

If any of these items is damaged or missing, please contact your dealer immediately.

2. Before using the overflow for the first time it is important to decide where to position the unit. It is best to have the overflow as far away from the inflow as possible to reduce turbulence. It is also important to check the capacity of your sump as well as the flow rate of your return pump on your filtration system to prevent possible overflow. It will be necessary to add water to the system after it begins running. The amount of water that you will need to add will depend on the sump size and design.

3. Assemble the bulkhead fittings as shown in the diagram. Insert the pre filter screens into the bulkheads and place the pre filter sponges over the screens. Insert the vent tubes into the holes at the top of the pre filter screens. The "O" rings can slide up or down to adjust the height of the vent tubes once the unit is operational.

4. Connect the bulkheads to the hoses going into your filtration system. You should use 1" Schedule 40 PVC inserted into the bulkheads. The PVC can be run directly into the filter or can be attached to flexible line using fittings. The fewest bends in the lines will ensure the quietest operation. To prevent leaking from where the PVC inserts into the bulkheads, you should use PVC cement to secure them into place (this, however, will make the connections permanent).

5. Place the unit on the edge of your tank and allow the water to fill the front chamber. Adjust the lower adjustment screws so that the unit is parallel to the back of the tank.

6. Adjust the height of the unit with the two top adjustment screws so the water level is about 1/4" above the edge of the front chamber. Make sure that the overflow is level with the water surface, not necessarily the tank, as some tanks may not be perfectly level. If you have small fish, adjust the fish screen so that the fish cannot enter the overflow. If you don't have small fish, discard the fish screen.

7. Using a container of water from your aquarium, fill the narrow chamber in the back of the overflow to the top. To start and maintain the siphon overflow, draw the air out through the nipple on top of the overflow by one of the following methods:

a) With a Venturi powerhead (highly recommended by CPR):

There are basically two different types of Venturi powerheads available on the market, Conventional and Rejuvenation. Conventional Venturi powerheads draw air in through a valve located on the outflow portion of the powerhead. If using this type of Venturi, make sure that the flow rate of the powerhead is at least 300-500 gallons per hour and that the powerhead is located near the surface of the tank. Rejuvenation Venturi powerheads draw in air prior to the impeller and are much more efficient. A flow rate of 200-300 gallons per hour is recommended if using this type of Venturi. These powerheads not only constantly draw out air which can stop your siphon, but after a power outage they can restart the siphon when the power comes back on. A check valve is recommended to prevent back siphon through the airline during a power outage. This, however, reduces the ability of the powerhead to draw air out from the overflow so a larger powerhead should be used.

b) Plugging the air nipple:

Use the airline that came with your overflow. Attach one end to the air nipple on top of the overflow and a check valve on the other end to prevent air from being drawn in through the airline. Draw out the air using a Venturi powerhead or your mouth.

c) Gravity siphon (not recommended by CPR):

Attach a long air line to the nipple on the overflow and, after drawing all the air out, place the end of the airline into a sump (which should be at least two feet below the overflow). This method is not as effective as those above.

8. When you initially start the siphon overflow, it will drain water from your aquarium down to the level of the front edge of the overflow box. The sump part of the filter should begin to fill with water. It may be necessary to add water to the sump if the return pump is not completely submerged. Make sure that the return hose is firmly attached to the return pump before plugging in the pump.

9. Plug in the return pump. Keep an eye on the water level in the tank to make sure the overflow starts its siphon. Water should start draining down the overflow. At this point, direct your attention to the sump to make sure the return pump is still completely submerged, or that the sump is not overflowing. It may be necessary to add or remove water from the sump at this point. Continue to monitor both the tank and the sump for a few minutes to ensure that the system is working consistently.

10. The water level in the aquarium can be adjusted by using the two top adjustment screws on the overflow. This is usually necessary once the overflow has been operational for a few minutes. By lowering the water level in the aquarium you will raise the level in the sump and possibly overflow it. By raising the water level in the aquarium you will lower the water level in the sump and the return pump could possibly burn out if not completely submerged. Add or remove water as needed.

11. At certain flow rates, there will be a flushing noise coming from the overflow. This noise is caused by air trying to escape back up through the bulkheads while the water is going down. The enclosed vent tubes will help reduce the level of noise. Place the tubes through the small holes in the pre filter screens and adjust them up or down until you find the point where the noise is reduced the most. Slide the small "O" rings down until they reach the pre filter screens, securing the position of the vent tubes.

Your overflow is now in operation.

The outside of your overflow may be cleaned with a non-abrasive cloth. Do not use detergent on any surface that comes into contact with aquarium water.

Got Rice?!?
2,648 Posts
HaHaHa, overflows just dont start initially by themselves. You have to get them going. Glad you got it working. Nate to the rescue.

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