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He does have some fish in it now and wants to add some crappie. Reason why this got brought up because I mentioned that crap will choke out all the oxygen if he leaves it as it sits. He'd waste $ on fish that would ultimately die.
Figured the rake was wear I was headed on this deal, but wanted to reach out just in case someone was more knowledgeable about a product.
Wait wait wait wait wait...
the plants will choke out all the oxygen? Seriously?!
Or chemically treating the Lemna sp. will destroy the oxygen values?
Before I say anything I need to know that you and your friend both know that even aquatic plants MAKE OXYGEN and don't "choke it out". The more vegetation in a pond the higher the DO content will be (in most cases), once veg states to die and decay, then the microbes that come in use the available DO to do their work and breakdown the veg.
Wait wait wait, you do realize duckweed is a floating aquatic plant? I would venture to believe that nearly all the oxygen being produced is at the leaf surface, so very little will make it into the water. Additionally, floating macrophytes limit the exchange between water surface and air, a significant source of DO, as well as reduce sunlight for submerged macrophytes and phytoplankton, the main source of oxygen. It is well documented that other species of floating macrophytes (salvinia, water lettuce, water hyacinth) have a significant impact on DO levels so I don't imagine why duckweed would be any different. So what you said it certainly true regarding submerged macrophytes but is incredibly oversimplified. Simply put, floating macrophytes will not increase the DO in a body of water.
This article below specifically addresses the effects of Lemna on DO but the gist of it is: "Anoxic condition resulted throughout the day in floating plant system. DO varied significantly with depths in oxic condition. DO dynamics in these wetlands can be explained by photosynthesis, respiration, deaeration and reaeration processes."
Lemna effects on DO