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Fluid Physics Question About Aquariums

  1. Jan 20, 2014 #1
    Hello, I have two fish tanks next to each other. One is big (36 gallons, about 2 feet tall), one is smaller (10 gallons, about a foot tall). I want to make a connective tube so that my eel can swim between the two tanks but I don't want either tank to overflow. I am not adverse to raising the shorter tank on wooden planks. Is there a way to make this work or am I doomed to failure? Also if my eel swims through it will it make a siphon and flood a tank?

    EDIT: The tube goes over the tops of the fish tanks, not through the walls.
    Last edited: Jan 20, 2014
  2. jcsd
  3. Jan 20, 2014 #2
    The water will want to be at the same absolute level in both tanks. Your best bet is to position them so that their upper sides are level. If not, you might be able to control the situation with a pump - but if it fails, things can get nasty.
  4. Jan 20, 2014 #3


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  5. Jan 20, 2014 #4
    Well, forget about this. If the water levels are a foot apart, and the diameter of the tube is just 3 cm, the application of Torricelli's law yields 1.7 l of water per second, so you would need a pump with the flow rate of 6 cubic meters, or 1600 gallons, per hour. That is significantly in excess of the recommended flow rate 230 gallons per hour for your tanks combined.
  6. Jan 20, 2014 #5

    With the glass wall top edges at equal height you will not overflow. Make sure that the siphon tube contains less water than the head space available in the tanks. The head space is height above the water but below the tank top edge.

    You can ignore the effect of the eel.

    You will have to fill the siphon and make sure that its lower edges are never above the the water surface.

    You are not using the siphon to transfer liquid but you may find this interesting
    Last edited by a moderator: Sep 25, 2014
  7. Jan 20, 2014 #6
    Thank you everyone! I am going to raise the smaller fish tank and I have confidence now that nothing will overflow. This is going to be a very cool setup that I have never seen before!!
  8. Jan 21, 2014 #7


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    You could always have the lower tank sealed, in which case you could have it at any height. It could be an interesting situation if it were actually a level below the bottom of the upper tank. If the joining tube went from the top of the lower tank to low down in the top tank, you could aerate into the bottom tank and the bubbles would flow up through the tube. You could use standard plumbing fittings, once you have the holes made in the glass.
    The eels might enjoy that and it would certainly be a conversation piece.

    Failing that, just keep the surfaces of the two tanks the same, using blocks. The eel won't make it all overflow or syphon.
  9. Jan 21, 2014 #8
    Thanks, I don't want to drill the tanks, that's why the tube is going over the tops. Plus I don't think it's practical to seal the lower tank since then I wouldn't be able to do maintenance on it. That is an interesting idea and if I ever make a fish tank, maybe I'll make a weird shape like that.
  10. Jan 21, 2014 #9


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    I get the picture now. If the pipe is full of water then the two water levels will always be the same (equalise, automatically) and the eel may just cause a small flow in the pipe, whilst swimming through, but the water will re-balance when he's through.
    The connecting pipe will need to be kept full of water in some way or bubbles will find their way in it and form an air barrier for the eel - although, if you give him some felt to squirm on, he can probably get over a small air filled hump at the top of the inverted U. They do go over land at times, I believe.
    How exciting. Keeping eels! Much more interesting than Koi.
  11. Jan 21, 2014 #10
    :D Thanks! If worse comes to worse I could drill a small hole in the pipe and install a pump to suck air out to it will fill back up with water
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