How do you know what pond weed/plants are good for a pond?

I've have a large pond %26amp; always beleived weeds/plants were good for the pond, i always beleived they put oxygen into the water, the other day i was speaking to a young lad who informed me that some weeds/plants put oxygen into the water in the day light hour but at night take oxygen from the water.

Is this true ? If so how can you tell

How do you know what pond weed/plants are good for a pond?
Yes it is true. Plants need light to manufacture energy. With out light it undergoes respiration like animals. The oxygen output is greater than its use though. Plants who floats, like water lettuce, hyacynth and even water lilies, would not really affect the oxygen level in your pond. Plants like Elodea and Valleria will. I had a very small pond with a crude filtration system, which is often turned off. It was so packed with Elodea, tha the fishes literally can't swim. The odd thing was the fish was at their healthies and happiest in that pond. The oxygen use for the fish probably decrease at night, and the individual plants probably uses very little. I figure the oxygens produce during the day probably is stored in the water to a certain degree. If your fish is happy and healthy, remember that mother nature doesn't worry about the plants in her pond uses oxygen at night. If your fish isn't doing so well, add a pump to increase the oxygen.
Reply:i think its true but im not posutive! :D
Reply:No it's not true...though if you have an inbalance of plants this can be detrimental to water quality. Some grasses are invasive and use up a lot of nutrients. So come the spring have a clear out before fish start spawning.



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Reply:all pond plants do this but it will not harm your fish, the more plants you have the better