I made an "overflow box" with household items.
Tell me what you think.
Used a water bottle for the overflow box. Made a siphon with a garden hose. Put the water bottle on a stand to get my water level where I wanted it (hard to tell from the pics but it's just above the wood trim in the tank and the water bottle). Drilled a hole in the water bottle just below the waterline and pressure fitted a hose. This hose drains into my sump. The sump can't flood because there is no siphon from the water bottle to the sump, just gravity feeds the sump.
In the sump I put the return pump as high in the water as I could. This way if my overflow box breaks siphon I won't flood my tank. The pump will move about a gallon of water into the main tank before it runs dry and that's not enough to flood the main tank (I tested this theory out to be sure). If I've thought this through properly the only thing that could happen is the siphon breaks and the pump runs dry and might possibly burn up. $20 pump vs 10 gallon flood? I'll buy a new pump.
Here is what it looks like all together.
Tell me what you think.

Used a water bottle for the overflow box. Made a siphon with a garden hose. Put the water bottle on a stand to get my water level where I wanted it (hard to tell from the pics but it's just above the wood trim in the tank and the water bottle). Drilled a hole in the water bottle just below the waterline and pressure fitted a hose. This hose drains into my sump. The sump can't flood because there is no siphon from the water bottle to the sump, just gravity feeds the sump.

In the sump I put the return pump as high in the water as I could. This way if my overflow box breaks siphon I won't flood my tank. The pump will move about a gallon of water into the main tank before it runs dry and that's not enough to flood the main tank (I tested this theory out to be sure). If I've thought this through properly the only thing that could happen is the siphon breaks and the pump runs dry and might possibly burn up. $20 pump vs 10 gallon flood? I'll buy a new pump.
Here is what it looks like all together.
