# CO2 into HOB Filter



## hiyamoose (Nov 27, 2010)

I read in a thread that too much CO2 in a HOB filter being used as a reactor/diffuser can kill the beneficial bacteria in the filter. Is this true?

Also, will the decrease in the beneficial bacteria affect/promote hair algae? I'm trying to determine the cause of a small outbreak I have (already cleaned substrate and do weekly water changes to keep phosphate down from dead bio).


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## bryanmc1988 (Apr 4, 2012)

yes it is true, to much co2 will kill off any good bacteria, i say make a diffuser for the co2 and leave the filter alone, best was is to just buy a diffuser on ebay or your LFS its like 5-10$ shipped not a bad price to keep your fish alive, you got to invest to play with the hobby dont cut corner's it only leads to bad news along the road


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## Diana K (Dec 20, 2007)

Lets think through the 'reduced bacteria population...' scenario... 

1) This is a planted tank, right? 
You are aiming for good, reasonably fast plant growth? (Adding CO2 makes me think so)
Therefore... Plants play a big role as nitrogen sinks in this tank. 

2) Fact: Aquatic plants in general can do a really good job of removing ammonia from the water. If all the ammonia is gone, they can also remove nitrite and nitrate. 
Question: What is your NO3 test showing? 

3) The filter is not the only home of the nitrifying bacteria. Perhaps half or less of all the bacteria actually live in the filter. The rest live all over the tank, not right in the light, but under the leaves, on the rocks and driftwood, in the upper layer of the substrate.

Combine ideas 1), 2), 3). 
The amount of nitrogen processing done by the filter in a well planted tank, with strong growing plants is less than 25% of all the nitrogen removal going on in the tank. 

Now some more ideas:
When you dissolve CO2 in the water this does not necessarily mean there is no more oxygen in the water. Both can be in the water. This suggests that adding CO2 to the filter, allowing the impeller to chop it up and blend it with the water may not be excluding the oxygen that the bacteria need. So, you might be killing some bacteria, but by no means all of it. Lets say half of the population that is living in the filter, at worst. (and remember that in the filter is only 1/4 of the nitrogen removers in the system)

OK, so 1/8 of your ammonia removing life forms can no longer live in the filter. There is now a tiny bit more ammonia in the water. Such a tiny bit that your ammonia test will not react. The other 7/8 of your ammonia removing life forms (the plants and bacteria that is in the tank, and the remaining bacteria in the filter ) are still alive and on the job. 
The bacteria in the rest of the tank will sense it, and will reproduce to the maximum possible allowed by that additional food source. But the plants are doing so much better because of the CO2 that they might grab all the ammonia missed by the bacteria that died, leaving little or none for the other bacteria to grow on. 

Net result: 
The tiny trace of ammonia blip that shows up (if it shows up at all) is gone so fast that it is not there to maintain a growth of any sort of algae. Other ammonia removers take over really fast, in a well established tank. So there is no on-going problem from killing bacteria in the filter. 
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Here is another issue, though:

Inconsistent CO2 may trigger algae. 
HOB filters get worse as they fill with debris, and the flow pattern may change even when they are not so full as to slow down, but just starting to accumulate debris. These changes may alter the amount of CO2 that gets dissolved, and this variable amount of CO2 can trigger algae. 

Much better to get a separate CO2 dispersal system, and keep it working at as steady and stable a way as possible. Do not combine a variable system (a filter) with CO2.


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## hiyamoose (Nov 27, 2010)

WOW! That's about all I can say. Thanks so much!

I moved the CO2 glass diffuser from the HOB intake and put in beneath an older Powerhead 201 I had laying around here from a previous tank. Positioned it at the very bottom of the tank in the corner, so the dispersal was just above the substrate and blowing back across the width of the water column. 4 days now, and I can't get over the growth already. I played with the strength of the 201 output to make sure the CO2 makes it around the tank. Lots of little bubbles but I can live with that. And I'm guessing there's plenty more CO2 I can't see being moved around. Might try Nico's sponge idea on the output and see how it does if the bubbles really impede the view. For now, I'm thrilled to say the least. Will continue to dose Flourish and iron. May not need the K and Ph I ordered if growth keeps going the way it is.

Don't think I would have taken the time to move the CO2 diffuser if somebody hadn't said it is a good idea. Should listen to my instincts I suppose.

Thanks again!


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