Aquarium plants tend to be fairly isolated in their aquatic habitats, which helps keep them safe from most plant diseases. However, there are a few issues that can permeate the security of the fish bowl, so you need to keep an eye on your watery ecosphere to make sure that nothing is quietly demolishing your plants right under your very nose. There are few types of aquarium plant disease to look out for, so your job should be pretty easy.
Cryptocoryne disease, also called cryptocoryne rot, causes small holes to appear in the centers of the leaves of aquarium plants. Left unchecked, these holes will spread and eventually break down the entire plant. If you detect this rot, remove all dead and decaying plant materials, clean the substrate in the tank and replace as much of the water as possible (called a "large water change"). This disease may be caused by excess nitrate, so get your aquarium on a regular maintenance schedule and clean it regularly.
Iron chlorosis causes the leaves of plants to turn yellow, then almost clear (like glass), and finally become very brittle and rot away. It is generally a result of an iron or potassium deficiency or too much fertilization with phosphate. Use an iron-rich fertilizer to remedy the problem and remove affected leaves. The plant should be able to repair itself with this help.
If the weather outside is excessively dry, aphids and other small insects may start colonizing your aquarium plants. You will be able to see small, dark or green specks on the plants, and the leaves will start to look wilted and crumpled. Once you spot the problem, remove heavily affected areas of the plants and rinse them in fresh water. Make sure that all of the insects are gone before you replace the plants in the tank.