On my 200 gallon 6x2x2 with sump, I thought long and hard about the various methods available of shifting water to the sump, But my main gripe with drilling the tank base, was all the hassle of plumbing through the stand, and the fact that this would have meant fitting two overflow towers into the back corners with a subsequent loss of internal space when I wanted to keep these back corners clear. So rear overflows were the order of the day in the end. After getting the back of the tank drilled it was time to think about a way to set the water level in the tank itself, The two drain holes are both 55mm in diameter to accept a Marley 40mm bulkhead fitting, with a 90deg bend afterwards to take water back down to the sump. The reason I opted for 40mm  overflows is simply that after looking at several tanks that use 32mm overflows, I noticed how high up the overflow hole the water level was, this worried me slightly because if I wanted to 'up' my circulation at a later stage, it was obvious that the smaller diameter hole would have trouble coping with the increased flow. I think the larger diameter will also reduce the gargling sound commonly encountered when using 32mm overflows, - so 40mm it was.

Building the weirs is quite easy using some Acrylic sheet. As the diagram shows,-

 Its a simple case of cutting two triangles of glass, plus an oblong section to make op the front plate. The tank back glass forms the back plate once the fitting is silicone'd in place over the outlet hole just leaving the top open for the water to flow over. The crucial thing is to get the top of the weir in the right position so that the water level in the tank is right. To do this I simply set the top of the weir about 3mm above the black decor strip around the tanks top edge. The water level itself will probably sit a couple of mm above this point as it needs to rise higher to tip over the edge.

In action these weirs should be able to shift large quantities of water quite quietly. To stop prized possessions being washed over the edges. I will be cutting a triangular foam filter that sits inside the weir itself. this will be made from very course foam to keep water flow at a maximum, and to prevent the possibility of blockage. The only drawback to this method is that the tank has to be positioned slightly away from the wall to allow space for the piping, but that's only 4", so nothing drastic there. The down pipes themselves are angled at 45deg down to the sump to soften the flow of water and reduce the whooshing sound commonly encountered by vertical stand pipes.

These are the finished weirs silicone'd into position.

Once the tank was filled it was time to fine tune the weirs. One common complaint that people have with both stand pipes and overflows, is that terribly annoying gargling or flushing noise you sometimes get. In an effort to get rid of this I made two additions to the design. 1 was the addition of an egg crate grill ( pic bottom left) that sits inside the weir itself which cuts down on the turbulence as water rushes over the edge, (not to mention stopping any animals going over especially snails which seem to love it in these areas). The other was the addition of two breather tubes at the back of the elbow (pic bottom right) which allows the unimpeded flow of air down the outlet pipe. This prevents the noise created by air rushing either up the pipe, or being sucked into the bulkhead opening inside the weir itself.

The breather tubes are simply added by drilling two 9.0mm holes at the top of the elbow where it exits the bulkhead fitting. then two pieces of 10mm soft pipe are inserted. the difference in size ensures a nice tight fit which will stop any leaks from forming. The length of the pipes should be about 6" long. By adjusting how far the pipes are inserted into the elbow and down the outflow pipe, you can tune the pipe till it goes silent in operation. Several visitors have commented on how quiet this arrangement is. especially considering my skimmer makes more noise, and that is very quiet being in the pump cupboard under the main tank. Seeing as I'm considering another house move early on next year, I will probably make two new bends with a single larger fitting that will accept a tap on top. with this, it will be possible to set very precisely the amount of air allowed to flow down the overflow in an effort to get a virtually silent action.