This might be frowned upon, but maybe look and see what is growing in water around you? I know you said desert, but there is going to be water somewhere. You could always take a sample of one of those, though again, the legality of that might be... in a grey area as it were.
Otherwise, I don't have many suggestions. There might be some coldwater reed type plants, but I imagine they will poke over the surface.
Remember that the tank your fish lives in is his or her's whole world. Ask yourself this question, and answer it honestly: would you choose to live there?