Hi and welcome to the forum
LOL, you don't need 20 pounds of steel weights to hold it down
You can buy aquarium weights or use old fishing sinkers (use grey lead sinkers not shiny silver ones). The grey lead fishing sinkers have a lead oxide coating around them and won't leach lead into the water. The shiny silver lead sinkers don't have the oxide coating and can leach lead into the water. Most pet shops in America are now selling lead free plant weights and these are shiny silver and can be used. But check the packaging to make sure it says "lead free". Just tie the weights loosely around the base of the plant (just above the roots) with some cotton thread and they will hold it down. When the plant has settled in you can remove the weights.
The main drawback to buying large aquarium plants is they need plenty of substrate to grow in. If you only have a shallow substrate (1-2 inches), it won't be enough for the plant. You can either build up the substrate depth to 4-5 inches and put the plant in that. The deeper gravel will allow you to spread the roots out a bit and the plant should stay put. You can use rocks or driftwood to make a section with deeper gravel.
Alternatively you put the plant in a plastic 1-2 litre container (like a small icecream bucket) that is 4-5 inches high. Put some gravel in the bottom of the container, spread a thin layer of granulated garden fertiliser over the gravel. Put a 6mm (1/4 inch) thick layer of red clay over the fertiliser. Dry the clay and crush it into a powder before using it. Cover that with a bit more gravel and put the plant in it. The clay stops the fertiliser leaching into the water. When the plant root hit the clay and fertiliser, the plant will take off and grow like mad.
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