I am not against the PVC idea, but you have to realize that it has some important potential drawbacks.
As I mentioned in an earlier post, you have to use a separate means to remove trapped bubbles.
The aqualifter is probably the best solution to this, but aqualifter pumps are not perfect and actually I have found them prone to failure. They plug up quite easily and reduce or stop sucking. I had to take mine completely apart once to get it back to working order. About every 2 weeks I had to blow into the uptake tube to clear it out. Maybe I had a bad model, I don't know - but realize this is a point of failure.
While it is true that "The siphon on a normal u-tube is lost with in 3 seconds of the water level going below it's range", the fact is that on a good U-tube overflow, it is designed to NEVER let the water go down this far. Usually there is a baffel present or an extension tube on the overflow which maintains a minimum water level, higher than the bottom of the U-tube. Also, the lack of corners in the U-tube means that air bubbles can travel through and out of the tube, so even if there are some bubbles entering, it will clear them.
I am coming from the experience of running a CPR overflow for 9 months, which is functionally equivalent to the PVC designs linked above in that it has 90-degree angles and requires an aqualifter or similar. I spent a good bit of time worrying that the suction from the pump would fail. I eventually bought a good U-tube overflow and have eliminated one more pump to plug in.
I realize that any overflow design can fail, but in general, I think a good U-tube is more reliable than the CPR/PVC overflow design since it does not rely on a pump to maintain the siphon.
If you want an extra-safe overflow, get a U-tube one and then add a hole at the very top connected to an aqualifter pump.
My next tank will definately be reef ready or drilled to eliminate this potential failure (and lower the risk of flooding).
