Turtles can eat water lettuce and Water Hyacinth with no issues at all. However, these plants have almost no nutritional value and as such, should be fed as part of a varied diet, in moderation.
Do you mean Pistia sp. by water lettuce? They make excellent floating plants, but they either die rapidly or outgrow your tank. Most turtles dislike eating it or avoid eating it after tasting it, but keep an eye on your turtle if it consumes a lot of it. It does contain oxalates, which may be dangerous, but it is generally a fantastic floating plant.
Water lettuce can grow fast.
ALSO SEE: Can Turtles eat Cilantro?
Can You Feed Water Turtles Lettuce?
Yes! The key is variety. You can give them water lettuce and red/green leaf lettuce.
If tortoises/turtles have eaten soil/mud-grown water lettuces or if their roots begin to touch the substrate, keep an eye on them since they absorb a lot of heavy metals in their mineral form, which can cause liver and renal problems. However, it appears that those produced in well-filtered and conditioned water are now safer.
Water lettuce is a lovely plant, but it will overflow indoor tanks eventually. However, I don’t envision them ripping it to tears; if anything, I’m concerned about them hurting the roots.
If there’s one thing I’ve discovered, it’s that it burns so readily. In an indoor tank with two modest UVB bulbs, I nearly fried some of it. This isn’t a “show-stopper” plant. Outside, it can only take a few of hours of sun per day.