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I'm using the following setup to pump water out of a space below the gravel (yellow) in my aquarium:

enter image description here

The tubes displayed as lines are 12/16mm silicone tubes. The pump is a 300l/h electrical water pump.

The problem with this setup is that the biological filtration creates nitrogen compounds. They appear in large bubbles under the gravel which are sucked into the pump. After days the pump starts to run rough because it's not made for handling the gas bubbles in the tube.

I'd like to keep up this setup which is ideal with the exception of gas bubbles reaching the pump. Is there any way to separate the gas from the water before it reaches the pump inlet? A component? A smart arrangement of tube?

The relative height of the pump to the water surface, suction end of the tube or buttom of the aquarium can be modified in order to achieve a solution. I tried to have the pump on the same level as the aquarium as well as a slight elevation which seems to increase the time until the pump starts running rough a little bit.

I'd like to discuss the engineering problem rather than the aspects of running an undergravel filter.

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I think the problem is not gas but, according to your diagram. the pump is too high and it is not getting a siphon feed. Or ,as noted sand and debris are blocking the pump inlet. I have used many undergravel filters but usually with air bubble lift. UG filters work especially well with salt water as a whole community of animals develop in the gravel and consume debris keeping the gravel clean. I prefer bubble lift because pumps ,even at the bottom of the tube tend to plug with sand and debris, apparently because of high flow rates .

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The gases under the gravel could be vented to the surface via an undergravel lift tube. The intake for the pump should be below that vent level.

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I'm adressing this question without specific aquarium experience, so some issues may escape me. If the pump needs to draw the water over the aquarium wall, you can't have an open chamber before because you need some vaccuum for the pumping action to work.

I see two ways:

  • manually vent the pump and the tubing every now and then
  • drill a hole in the side of aquarium, duct the tubing horizontally to the pump and then have a larger, vertical vhamer (a piece of pipe, DN32 should be enough) that up to above the water level, that should allow venting of the nitrogen bubbles. This wont work ifthe pump has any lifting to do on the suction side.
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