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!