DIY Small Tank Overflow - PVC Piping

Piping Diagram

The diagram below shows the plumbing as if it were laid out flat. From this diagram you can see the basic assembly.

Parts Needed (Sch 40 PVC Pipe):
(1) 2" Coupling
(1) 2" Plug
(7) 3/4" Elbows
(2) 1" Caps
(1) 1" x 3/4" Tee
(1) 1" Elbow (Optional depending on return plumbing)
(1) 1/8" Air Line Check Valve
5' of 3/4" Pipe
1' of 1" Pipe

Assembly

1. Start by drilling a 7/8" hole in the 2" Plug as close to one side as possible. This allows for the insertion of the 3/4" Standpipe. The reason for this instead of using a reducer is that it allowed me to get the standpipe closer to the corner of the tank.
2. Cut strainer slots into one end of the 2" coupling. This can be done with any number of tools. I used my table saw. It is very important to be careful with this step because a stray hermit crab or snail can plug up the piping and cause a tank overflow. If you have these animals in the tank, you may want to fashion some kind of cover for the strainer - possibly out of eggcrate - just to be safe.
3. Push a length of 3/4" pipe into the hole in the 2" plug and glue it to secure it if needed. I did not use any glue on my assembly - everything fit tight. When I assemble PVC that will not be under pressure, I use a little PTFE (Teflon) tape on the ends of the pipes to tighten the joint and prevent leaks.
4. Slide the uncut end of the coupling onto the plug to create the overflow strainer.
5. Drill a 1/8" hole into one of the 3/4" elbows as shown below, and jam the inlet end of an air line check valve into the hole. To be safe, you may want to add a little silicone sealant to this joint. You will use this to suck the air out of the piping to start the siphon.
6. Assemble the fittings as shown above. In my tank, the piping goes down one corner, across the tank (under the sand) and up the opposite corner - it looks a little more intentional that way. For now, leave off the 1" cap - I'll address that later.
7. The lower you can make the 1" x 3/4" Tee, the higher the capacity of the overflow piping will be (water will run through the piping faster because the "outlet" will be lower than the tank water level). However, it must be above the lowest pipe in the tank to prevent loss of siphon if the fill pump turns off for any reason.
8. Finally, connect a hose (1/2" ID or larger) to the end of the piping, or hard-pipe from the 1" Tee back to your sump - I'll leave the details of that part to you.
9. Drill (4) 1/16" holes into the top of one of the 1" caps (save the other for starting the overflow). This will be used to allow air into the system to prevent the plumbing going back to the sump from sucking all of the water out of the pipe and breaking the siphon. If you like, you can drill a few extra holes and stuff some filter fiber into the cap to make a silencer. In general, this is a pretty quiet overflow, but there may be a slight sucking sound coming from the holes in the cap.

Starting the Overflow

To start the system, you will need to have water in the sump and in the tank high enough to fill the overflow strainer.

Replace the 1" cap with the second cap that has no holes in it. If the outlet into the sump is not under water, this may be a little harder, because you will need to block the end and release it as soon as the water starts flowing through the overflow. My recommendation is to get the end of the pipe or hose into a bucket of water somehow.

Once the cap is replaced and the end of the outlet is underwater, suck the air out of the piping through the check valve. Feel free to use a piece of airline to accomplish this - there is no need to bang your head on the wall in an attempt to reach the check valve with your mouth. Once the air is completely sucked out, the overflow will start running and the tank level will drop - you should immediately remove the 1" cap to prevent the siphon from breaking when the tank water gets below the overflow.

Rear View