The 125 gallon tank has 2 overflow boxes that use a 1” siphon tube. They were purchased from Ron’s Saltwater Heaven for $25 including bulkhead fittings.  I think that the boxes are rated for ~300 gph.  I like this overflow box design because it has a secondary overflow point on the outside box, which means that the length of the siphon tube on the outside box does not really matter.  The box inside the tank has ¼” gapped teeth that allows for surface skimming of the water. Some people do not like having these type of boxes in the tank because of appearance, but if they are set to the correct flow rate the siphon will not break, therefore they are reliable.  The reason the siphon does not break is because the water velocity through the siphon is fast enough to sweep any bubbles through the pipe.  Other designs have other methods, albeit less reliable, to abate this problem (see the warning below).

Stopping the Noise

The inherent problem with all overflows, including overflows built into the tank, is the gurgling sound that they create.  The noise is created by fast flowing water entering a small diameter tube.  The velocity is such that air is sucked into the tube creating a vortex and lots of noise.  I had been battling this problem with a vengeance because my tank sits in the living room of my house.   At one point it was so bad that you could hear the tank from upstairs.

Richard Durso offered a simple solution on his web page (original design) that he posted on the reefkeepers newsgroup.   Unfortunately, I could not utilize his original design because it was not compact enough to fit into an overflow box.  Therefore I devised the following solution: 

                        Flow Diagram                                                                                 In the 75 AGA overflow   

In an external overflow                                 unassembled                   partially asembled            assembled

The Silencer:

What you need:

Tools: Dremel; drill, miter saw (optional), tubing cutter or hacket saw

12” piece of 1” PVC

2” PVC union fitting

2” to 1” reduction fitting

1” end cap with a 1/8” hole drilled in the top.

Construction: 

1. With a Dremel tool, sand paper, or a knife, grind down the lip of the reduction fitting on the inside of the 1 portion.  This will allow the 1” pipe to slide into and through the reduction fitting
2. Align the 1” pipe with the reduction fitting so that ~1” of the tube sticks through the top of the fitting. Mark the pipe so that when holes are drilled in the pipe they will be above the 2” union fitting.  This is important. Notice in the picture how none of the holes are visible after assembly.
3. Drill the holes in the pipe with a 3/8” drill bit.  Then slide the tube into the reduction fitting. Alternatively, use a miter saw to cut an opening in the side of the pipe that is 1" by 1" (see the flow diagram above). This is my preferred method because there are no small holes that can be clogged with debris.
4. Slide the pipe into the reduction fitting so that enough of the pipe sticks out to fit on the end cap.
5. Drill a hole in the top of the end cap such that the hole can be fitted with a plastic needle valve (from That Pet Place for ~$1).  Slide on the 1” cap.  I have used a needle valve, but have found that just a couple of 1/8” holes work just as well.
6. Slide on the 2” union fitting.
7. Gluing is not necessary.
8. For internal overflow, I use a sliding fitting ( $7 at Lowes or Home Depot) to adjust the height in the overflow. This allows one to adjust the height by about 4 inches. Not necessary, but this eliminates knowing exactly how long the thing has to be.

Warning:  With the 3/8” hole drilling in the standpipe, algae can get stuck in them if the pass though the overflow.  If I notice the water level a little high in the tank, I clean out both standpipes.