If I was attacking this problem this is what I would try,(Water Hyacinth) especially since you live in hot Texas. They might take over your pond but that is what I would be after right now.
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The most popular floater is the Water Hyacinth (Eichhornia crassipes). This rapidly multiplying plant develops a strong trailing root system which removes excess nutrients and pollutants from the pond. Due to their natural filtering ability, they are commonly used for water purification at treatment plants in the southern states. Occasionally hyacinths will develop a lovely orchid-like flower. ======================================================================
I also like the idea of water cress. I once went to the grocery and bought a bunch, then just push them into the edge of the pond with my fingers and they grew. And perhaps some submerged plants to help aerate the water.
