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I have always wondered how much plants actually help with oxygen in the tank, but I can't seem to find a good answer. I have seen aquariums fully stocked with plants with little to no agitation going on. Do plants really help to the point where agitation isn't needed? I'd like to know this because my cousin, who is new to all of this, has a 2.5 gallon tank with a single Molly fry (will be upgrading), the filter has a very slow flow and there's almost no agitation from it. For that reason I told him to get plants, so he started with 4 Amazon Swords. The Molly doesn't seem to be gasping for breath at the surface and swims around very actively. I also have limited the flow in my community tanks too. To sum it up my questions are: How much do plants actually contribute to oxygen? More than filter outflows? Can you have a tank that has no agitation if its heavily planted? Thanks!
 

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you don't need agitation in a tank regardless of plants. Fish do not require much oxygen at all and difussion from the surface will usually suffice. Also the water can only hold so much O2 to the point where all the agitation, bubbles, and plants will have no additional effect on it.
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To me what is important is the plants removing the carbon dioxide and ammonia as well.

I have ran planted tanks with no mechanical circulation or filtration for up to 9 years with a very heavy bioload and descendants of the original cycle fish.

In those tanks I have a very high ph (8.4-8.8 api high range test kit). Yet even fish reported to need much lower pH valus thrive for years and years. I suspect this is because the plant action has made the tank a net consumer of co2 and producer of oxygen each day. If that is true, adding circulation would actually decrease the oxygen and increase the co2.



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I believe plants add a lot of oxygen to the water (some of it is used at night). I can actually see streams of fine bubbles coming out of leaves during intense photosynthesis. Some plants are "pearling" oxygen if light is over 3-4 w/g, CO2 and right ferts. Water is oversaturated with oxygen, it cannot dissolve any more, the entire tank looks like a glass of champagne.
Surface agitation is detrimental to aquatic plants because it helps dissipate little CO2 that accumulates from decomposition, fish respiration, bacteria activities, etc.
 

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I believe plants add a lot of oxygen to the water (some of it is used at night). I can actually see streams of fine bubbles coming out of leaves during intense photosynthesis. Some plants are "pearling" oxygen if light is over 3-4 w/g, CO2 and right ferts. Water is oversaturated with oxygen, it cannot dissolve any more, the entire tank looks like a glass of champagne.
Surface agitation is detrimental to aquatic plants because it helps dissipate little CO2 that accumulates from decomposition, fish respiration, bacteria activities, etc.

Actually I get pearling at 1.5-2 watts/gallon.
 
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