Why are my aquarium plants getting black rotting marks on them?

Ive had them for around 4-5 months now. They been doing good for a while, but now all of a sudden some of the plants are rotting away. Going from healthy green leaves to black marks and spreading. The leaves dont even turn yellow, they go from green to those black marks. I have them in a 20 gallon tank with a bunch of orted small fish and keep the tank lights on for more than 8 hours. They are in normal gravel, nothing special. I do 30-40% water changes every 11-13 days. The black marks started on my java ferns and now my other plant is getting it. I use just a bio wheel as a filter. No air pumps or anything else. Anyone know whats wrong?