Water Lettuce will rot in humid environments. I cannot be kept under coverglass and will rot if it gets water splashing on it from a filter or fountain. It likes lots of light and dry leaves.
Elodea rots if it is exposed to a sudden change in temperature or water chemistry (pH and GH).
Java Fern from plant suppliers is grown out of water and the terrestrial leaves usually rot when the plant is kept underwater.
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Plants need bright light and a bit of plant fertiliser. I used Sera Florena liquid iron based aquarium plant fertiliser and monitored the iron levels with an iron (Fe) test kit. I kept the levels at 1mg/Ltr (1ppm) and gave the plants plenty of light. They did well with that.
If you have 1 light unit on the tank, try adding a second light unit and see if it helps. Most of my plant and coral tanks had 4-6 fluorescent lights on them.
Make sure the lights are on for at least 12 hours a day, and anywhere up to 16 hours a day.
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You can try growing the plants outside in plastic storage containers without fish. Add some fertiliser and give them full sun and see how they do. If they get covered in algae, shade them during the middle of the day from 11am-3pm.
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Try different plants like Ambulia, Hygrophila polysperma and ruba/ rubra, narrow Vallis and Amazon Sword plants. Get some Water Sprite for the floating plants.
I have never been able to grow Water Lettuce in an aquarium and I don't know anyone who has.
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You don't need carbon dioxide (CO2) for aquatic plants. There is heaps being produced 24/7 in aquariums by the fish and bacteria. There is also plenty of CO2 in the atmosphere and it gets into the water.