Tilapia are plant eaters and usually feed off algae that they filter from the water by using tiny combs inside their gills or duckweed they find on the surface of the water. Duckweed can be combined with commercial fish feed to raise the protein level in their diet. You can grow duckweed to feed tilapia by introducing it to a water source and allowing it to flourish in a sunny area until it's grown and ready for collecting.
Find a quiet stream or pond that you can remove the duckweed from without any consequences. Go to the water source and locate the duckweed on the water's surface. Look for thick mats of green floating on the surface. Duckweed is small and green with no stem or leaf structures.
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Scoop the duckweed out of the water with a net and place it in a 5 gallon bucket with just enough water to keep it moist. Continue until you have the desired amount of duckweed. You will need enough duckweed to feed the fish and still have enough left over to grow and repopulate the supply. If necessary seek out more duckweed deposits if there are not enough at the first location.
Using the net transfer a portion of the collected duckweed to the water that harbors the tilapia you want to feed. Duckweed receives all of its nutrients from the water source and sunlight and it will grow best on still water in a sunny area.
Transfer the remaining duckweed into an aquarium filled with water from your tilapia pond or pool. Place the aquarium in a sunny area and check it daily, replacing the water as it evaporates. New healthy fronds of duckweed will grow in the aquarium and can be added those in the tilapia pond or pool to keep your crop healthy and encourage new growth. About once a month take half of the fronds from the aquarium and add them to the tilapia's water source to help keep the duckweed population healthy, reproducing, and providing food for the fish.