In that part of Texas, you'll soon see there's a natural seed bank. When your pond fills, it will take a little while, but you'll see plants begin to grow. My faves there are: American pondweed, or any of the other leafy species of pondweed. You're likely to get bushy pondweed, (Southern Naiad) and others. Bushy pondweed can get a little too thick and invasive, depending how your shorelines are. Shallow water can grow lots of it in thick masses.
If you want to buy plants, look at eelgrass, Vallisneria. It's a good one, too.
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