Water turtles LOVE to eat water plants. I've found that they particularly love the water hyacinth. They eat water lilies too, but they wreck the plant by swimming through the stems and breaking them off. But they'll eat all water plants.
Here is a list of plants turtles like to eat;
Anacharis
Elodea
Water lettuce
Hornwort
Cabomba
Water fern
Water hyacinth
Water lilies
Lemna minor
Lemna minor (common duckweed) is high in calcium and protein. I typically recommend feeding these as most turtles accept it and is very easy to grow and grows rapidly so when turtles eat it it will grow back again.
A question for a friend with RES: Is there a good place to buy the above listed plants? Seems silly to buy duck weed, but we are still 6-8 weeks away from spring weather here.
Mine ate all the water hyacinth, floating water lettuce, and the duck weed. For some reason they never touched the water lillies. They also ate all the anacharis until there was none left.
cattails (Typha sp.), sagittaria (Sagittaria subulata), val (Vallisneria sp.), marsh arrow head (Sagittaria cuneata), mud plantain (Alisma subcordata), water lilies (Nymphaea odorata), floating hearts (Nymphoides sp.), various marsh grasses like switch grass (Panicum virgatum) and cord grass (Spartina sp), rushes (Juncaceae), reeds (marsh bamboo Phragmites australis) Ludwigia repens, golden club (Orontium aquaticum), cranberry (Vaccinium macrocarpon) and blueberry (Vaccinium corymbosum) fruit are all edible.