Turtles, koi and goldfish do tend to chow down on the delicate submerged aquatics when they go looking for nibblies. Anticipating their grazing makes some aquatic plants more suitable for ponds than others.
How plants grow in an aquarium is going to be different to how they cope outdoors, where coping with far more volatile temperatures, water quality and critters is a big factor, pond plants are well adapted and 'hardened off' compared to the soft tissues of delicate plants better adapted to nice cosey steady aquarium conditions
Hornwort and Cabomba have a 'I'm not so tasty' feature which improves their survival
Anacharis has a fast growing habit, which tries to keep up with critters. Fortunately it has a simple root so it is easy to control when it masses up. Bladderwort makes a soft mass which seems to be pestered less by grazers. Hairgrass has deceptive graceful elegant looks, but very very tough. Ludwigia has dramatic colours where it has good light.
Elodea tends to root in and is far more difficult to weed, smother waterlilies, that is always off my agenda for ornamental ponds
Regards, andy
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