Water flow and light is not your problem. My experience with duckweed, slvinia, and red root floaters is that flow doesn't kill them. Even when fully submerged for extended periods time they still grow and produce oxygen.
The primary reason for no or slow growth or death of floating plants is lack of nutrients in the water. You probably need to fertilize the water. Preferably try a fertilizer without micro nutrients and macros except for nitrogen or phosphate. Hopefully that will allow the plant to grow.
Slavinia and red root floater are also good choices for floating plants. Don't get dockweed. it is very difficult to eradicate from an aquarium when you don't want it any more. If you miss one tiny speck of it, it will grow back.quickly. Focus on larger plants which can easily be removed when you don't want them.