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You might be able to buy some duckweed from pet stores in your area. Otherwise, you can take it from a pond it's growing in. Fill your container with some water from the pond. Then scoop some duckweed from the pond with your hand and place the duckweed into your container.  Place the lid on the container to prevent splashing while you bring the duckweed home. Between 50-100 duckweed pods will be enough as they will grow and multiply quickly. You'll need potassium permanganate to disinfect the duckweed. Mix one teaspoon of this chemical in 12 gallons (45 L) of water. Place the duckweed into the permanganate solution for 30 seconds or so.  Potassium permanganate can be bought at your local pharmacy. Disinfecting the duckweed will make sure that it is free of pests and bacteria. This will help you create a suitable environment for the duckweed to thrive in. Fill the tray with freshwater and add the duckweed to it. Use freshwater from a pond for best results but you can also use tap water. You need to use freshwater as duckweeds  are freshwater plants. This is why they only grow in ponds and not at sea. Salt water will kill the duckweed. The best place to put the tray is right next to a window that receives at least 10 hours of sunlight each day. Like many plants, duckweed will thrive in direct sunlight. If you see duckweed in a pond, notice how the pond is almost completely without any shade and receiving plenty of sunlight. If you can't put duckweed near a window that receives 10 hours of sunlight, you can also use fluorescent light bulbs to help them grow. For best results, place the light bulbs 15 inches (38 cm) above the tray. The duckweed won't grow under these light bulbs as well as it would with direct sunlight. Check the tray a couple of times a day and remove damaged duckweed from the tray. Replace the water in the tray with more freshwater. It will take 10 days or so for the duckweed to multiply. If transferring the duckweed to a pond in your garden, make sure that the pond is receiving plenty of sunlight. If you're moving the duckweed to an aquarium, make sure that the aquarium lid has a light source attached to it.  Duckweed requires no further care or maintenance once it's in your pond or aquarium. You can get a suitable net in your local garden store.

Summary:
Buy some duckweed at a pet store or harvest it from a pond. Disinfect the duckweed with potassium permanganate. Place the duckweed in a plastic tray with 12–14 inches (30–36 cm) of depth. Position the tray so it receives 10 hours of sunlight daily. Change the water in the tray after 1 week. Use a net to transfer the multiplied duckweed to your desired location.