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Discussion Starter · #1 ·
I made 2 home made sponge filters the other day and the first one has a riser tube about 6 inches above the top of the sponge, leaving me with only about 3 inches left of this tube.

So my first question is how far above the top surface of the sponge does the tube need to be to create the proper flow of water with an airstone?

2nd ? - When not using the sponge filter does it still need an airstone running in in to remain seeded or can I just stick it behind the driftwood in my main tank til i need to use it again?

Thanks in advance.
 

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Half of that, about 3" would be plenty.

Yes, it'll retain some bacteria, but not as much without the water flow through it. Hard to say if it'll be enough, but I do this all the time. I leave unused sponges in my sumps where there's a good flow of water.
 

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What determines how much bacteria stays alive and well is the amount of water flowing through the spong. If no water flows through, there is no O2 and they will die.
 

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But the whole sponge won't be anoxic. Water can flow around it and it'll harbor bacteria in the outer layers still.
 

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That's not really true Bill.

Air bubbles rise at the same rate through water no matter what size or length the tube is. The only thing that affects their velocity is the size of the bubble (large bubbles rise faster than small bubbles). Since the rise of bubbles and the ensuing pull of water they create is what causes the flow there is no difference at all in water flow created by tube height.

Since the tube outlet is closer to the water surface there will be less diffusion of flow at the surface causing more surface agitation. Think of standing right in front of a fire hose and then standing about 50' away as an example. The flow is the same, however the velocity and pressure is much higher closer to the nozzle.

Personally I like to keep my outlet tubes trimmed about 2-3" above the sponge.

Andy
 
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