any pond plant releases oxygen during the day but CO2 in the dark. I suspect you mean ones like Myriophyllum aquaticum and Elodea Canadensis. these are both ideal for small ponds. for larger ponds/lakes there are other plants like water crowfoot [Ranunculus aquatilis].

the rhs has a list of aquatic plants.

Many of them grow very rapidly, particularly Elodea so you don't need a lot to start with! If your pond is a small one I could send you enough to get you started.