Resa, I see you are in Louisiana. Anarchis, cabomba, duckweed, water hyacinth, hydrilla, lagarosiphon and water lettuce are all commonly found in ponds and streams.
Everything except for the first 2 are considered invasive weeds, which means you can pull out and keep as much as you want (just don't put any back!) There may be a limit to how much anarchis and cabomba you can gather - you can ask about it anywhere you get a fishing license.
Generally, the limit is for amounts you could re-sale. You can grab a few strands for your fish tank just fine, but you can't pull out a few 5 gallon bucket's worth.