Black Spots & Discolouration.

It's always tricky to diagnose plant problems .............

How hard is your water? Dark spots can be down to shortage of Calcium or Magnesium which are lacking in soft waters.

Sometimes when algae grow on plants, the part of the plant underneath the algae dies from lack of light. When the algae itself later dies, the dark dead spots on the plants are revealed. In this case, the dark areas were there before but just weren't noticeable because they were covered by algae.

"Reducing the amount of light does not seem to have worked either". Plants need enough light to grow. Algae can usually survive at lower light levels than other plants and so reducing light levels is more likely to harm the plants than the algae. I'd try increasing light duration to 10 - 12 hours a day, though you don't say what it is at present. Fast growing plants can outcompete algae.

That's fine, it's about the same as mine. Tropical plants would receive about 12 hours a day in nature and so they should thrive in the same conditions in an aquarium.