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I've tried feeding the following list of aquatic plants, and probably others I can't think of. Some he'll tear at making a mess, but not eating.

Anacharis
Amazon Sword
Water Lily
Duckweed
Japanese Peneywort
Water Hyacinth
Hornwort

so far the one he does eat is Frogbit. I could grow just Frogbit in the tank, but that is such a ware of the tank's real-estate and growing equipment.

oh and snails - ramshorn snails - Thor definitely likes them, so that's it two items.

I really want to grow another plant!

please somebody, what aquatic plants (besides those listed above) will your RES eat?

Could you grow the Frogbit in a separate container? I don't have anything to add but I'll love to see if there are others worth trying.

How about Elodea? My musk turtle, which is from a mainly carnivorous species, even eats it.

As far as RES go, mine aren't too happy with plant food either. They do like some water hyacinth now and then, but that's about it. Oh, well. At least the pond doesn't end up looking like a war zone.

If your turtle refuses to eat vegetables of any kind whatsoever, I recommend trying Hikari Excel for Cichlids or algae wafers. They have the pelleted form that most turtles like, but they have a good amount of vegetable ingredients too.