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In line with BillD's recommendation, most stem plants are good for nitrate absortion. One thought comes to mind and that is what is your level of nitrates? Is your tank fully cycled?
Good thing I asked. There is nothing wrong with having that level. 20ppm is not a problem. The other issue is, plants will need nitrate to grow and stay healthy. 5ppm will be used up pretty quickly and cause your plants to suffer.
I have learned that the hard way. My planted tank is constantly at 0 nitrates and some various types of algae have taken advantage. I will begin dosing nitrate very soon (waiting for a paycheck ) While plants are effective at lowering nitrates, algae will attack if you do not provide all the nutrients your plants need to grow. As I said, I learned that the hard way.