This device would go in your upper small tank.
Instructions from the AKA:
A dripping inlet is easy (use drip-irrigation valves), but an overflow that is reliable is a big problem for many. Bulkhead fittings weaken the tank structure, and can make the tank less useful for other purposes. The accompanying figure shows a simple arrangement of CPVC pipe and fittings that will allow the tank to overflow and keep the exact waterline shown.
The parts list was one tee and 5 elbows with a few pieces cut from a 5' length of 1/2" CPVC. I meant to have two tees, but got home to find only one in the bag. The other was to go crossways on the inlet, where it could be stuffed with floss to prevent loss of babies.
Without the tee shown, this structure would just drain the tank to the level of the inlet, as the siphon keeps running. When the tank water level gets even with the horizontal section between the last elbow and the tee, air can rush in to fill that horizontal section and the outlet pipe.
Why does it work? It may not be obvious, but it is simple. Look at it from the start. If you had only the first two elbows and two straight pieces of tubing, filling the tubes would siphon the tank down to the level of the inside pipe opening, if it was higher. Since it is not, the siphon would be broken when the water dropped to be level with the outlet. Air would rush into the vertical outside tube and the siphon would be stopped It would need restarting the next time.
Add the next two elbows and a third vertical section, and the siphon now stops at the level of the top of the third piece of vertical pipe, but air cannot rush in to empty the part running up over the rim. The tank water is level with the upper end of that pipe. Now one can add more water to the tank, and it will run out until the level is back to the same point. It needs only to be started once.
The rest of the system is to just get the water out of the house without wetting the carpet. An elbow and tee send the water down the pipe, but still let in the air that stops draining the tank at that level. Dumping a big pitcher of water into the tank did not threaten to overflow at the tee. Water flows less freely than air and we are taking advantage of that. Unless the inlet filter gets truly clogged, there is no reason to expect the tank to overflow when a trickle of water is allowed in.
The inlet can use one of those perforated conical caps that outside filters use, or it could be a Tetra Billi sponge (drill side holes in the inlet pipe), or it could be a tee filled with filter floss. The tube shown here does not reach the bottom of this tank, as it was cut for use with a 5G tank. Having it low in the tank allows catching more debris, but requires more careful attention that the inlet filter is not clogged.
The one in the picture was made of small CPVC and folded to lie flat between the tank and the wall. It was pulled out to photograph it from an angle that showed it all. Put a thumb over the open end of the tee to start it as a regular siphon. Release it to stop the draining at the level of the tee-elbow pipe.
