Without any other information it is difficult to say what might be casuing the decline. However, if temperature for your lowlanders is your concern, I would just add a small (or large - depending on your tank size) aquarium water heater to a pool of water in your tank. You can raise your plants on some eggcrate, with the water and heater under the eggcrate. If this is not an option, you can get a water container and a tiny water heater that can you can place next to your plants. I do this for both of my Lowland tanks and it keeps the temperatures at night perfect. I would suggest you get a decent water heater with a temperature control so it turns off automatically. Also, make sure you place it in a sufficient amount of water. How big is your tank, and how much extra space do you have? Water placed underneath the eggcrate is nice, because it elevates your plants closer to the light, and only affects the vertical space.