Java ferns are aquatic plants that grow well in fish tanks. Naturally found in tropical regions of Southeast Asia, the Java fern grows in shallow areas of streams and rivers. These ferns can grow to about 8 inches in height and will wrap around rocks, wood and other ornaments in an aquarium.
Though Java ferns grow well in moderate light and even in some shade, they need a certain amount of light to remain healthy. The ideal amount of light for a Java fern is at least two watts of light per gallon of water in the aquarium. The light should come from full-spectrum bulbs. If they do not get enough light, their leaves may begin to darken and the plant may eventually die.
Java ferns grow best in aquariums with certain alkalinity and pH balances. If your plant is turning black, purchase a kit to test these levels in your tank. For the best growth conditions, the alkalinity of the aquarium should be between 3 and 8 dKH and the pH should be between 6.0 and 7.5. The water temperature should be between 18.3 and 27.8 degrees Celsius.
Nutrient Deficiencies or Plant Rot
Because these plants grow on wood, rocks or other aquarium ornaments, Java ferns do not take up nutrients from the substrate of the aquarium. If the leaves of the Java plant begin to darken, it may not be getting enough nutrients. An aquarium fertiliser should be used to prevent malnutrition in your Java ferns. Java ferns are also subject to rot when planted in the soil, sand or gravel at the bottom of aquariums. The roots should be allowed to wrap around rocks or wood. The fern may be tied to the anchoring object with fishing line until its roots take hold.
The darkening of a Java plant's leaves does not always signal a problem with the plant. If your plant's leaves begin to darken or get small dark spots, it may be reproducing. During reproduction, dark spores form on the leaves. New Java plants form in these spores and begin to grow. They are released from the mother plant while they are still small and will then begin to grow on their own.