Tank above tank - THIS IS HOW!

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Simplicity is the best way!

I just tested a theory I had, using an old whisper air pump and a spare 10 gallon tank. Here's what I did:

The pump:
I connected a small bore tube to the air intake of the whisper. (Luckily, the one I have has a rubber base with a single air intake hole.)
I sealed the pump with silicone, all round the mains cable and air outlet nozzles as well, so that the air only had one way in.

The air intake was then connected to a length of 6mm air tubing, with a check valve inline, long enough to go from the pump to the bottom of the tank. Another piece of small bore pipe was then pushed into a suction cup and the tube attached. This was then placed as close to the bottom of the tank as I could get it.
I connected the two pump outlets together with a tee piece. These were left un-connected to anything, to vent the air outside the assembly.

I turned the tank upside down and rested it on the top of my 4' tank, resting it on the stress bars.
When this was complete, I connect my hose pipe to the shower, so as to get the temperature right and carefully added water to the tank until it covered the bottom lip of the up-turned tank.

Then I connected the pump to the mains and waited.
Very quickly the water level started to rise in the tank. I had to keep topping the water up in the tank to make sure the lip stayed under the water. It was amazing! Within ten minutes the whisper had drawn the water to within two inches of the top of the tank. I put an airstone on the outlet and put it under the rim, so the air went back into the tank rather than escaping, and the water level stayed where it was!

I have been looking in the wrong direction! It's not the weight of water that is the problem, it's how to extract the air from the top. The water is supported from below by the rest of the water in the bottom tank, and it takes very little suction to get the column up the tank! It is also near impossible for the bottom tank to overflow, even though there is 10 gallons more in it, as if the water level drops below the lip of the top tank, a large air bubble shoots in and the water displacement restores the level to seal again! Even if the pump fails, the water can't get out. The only way this could overflow the tank is if there is an air leak, or one of he pipes becomes disconnected.

I like it, and so do the fish! They have already started going up there.

It's a very neat and tidy way to increase the capacity of your tank too! I had a 40 gallon, now I have a 50 gallon and it cost next to nothing! Now I need to build a tank to suit my new 8' so I can do the job properly! I'll let you all know the results later.

I have just uploaded a video of the setup

Here's the proof:
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