PVC pip 2" or less in diameter would soon be covered by dead organics and other wind blown in materials. I would just lay small diameter pipe on the bottom. If sealed it will want to float so some weight would be needed to hold it on the bottom. Hose 5/8" ID needs one half pound of weight per foot to hold it on the bottom.
I would not add organics to the pond to create a plankton bloom. Use fertilizer prepared for pond use. But first before fertilizer measure the alkalinity. Water alkalinity needs to be above 35 and better at 50 parts per million for good plankton growth. Otherwise fertilizer does not work. Before you start fertilizing a pond read through this found in our Common Pond Questions section (Q&A) .
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