There are two common species of plants that go by the name of water sprite. One is Ceratopteris thalictroides and the second species is C. cornuta.
Q. My water sprite grows fast in my planted aquarium, which is to be expected, but there seems to be a dark kind of tar growing on it. It begins at the base, grows up a few stems and then out to the leaves. The leaves seem to be wilting and dying. What is going on here? Any help would be much appreciated!
A. There are two common species of aquatic plants that go by the name of water sprite. One is Ceratopteris thalictroides, which comes in two varieties. One has very fine leaves, and the other has broader leaves. The second species is Ceratopteris cornuta, which has even broader leaves than the broad-leafed Ceratopteris thalictroides. All of these have the same fast growth and high nutrient needs. They will also occasionally develop finely branched growth: spiky leaves that are the reproductive fronds.
The similarities of Ceratopteris. cornuta and the two varieties of Ceratopteris thalictroides, along with the two different leaf structures that each develops, has led some folks (including dealers) to refer to all of these plants as water sprite. Ceratopteris cornuta does not grow well unless it is floating, whereas Ceratopteris thalictroides grows well floating or planted in substrate. However, it will grow fastest when floating, where it is close to the lights and has access to the ample carbon dioxide (CO2) in the air.
If your water sprite is planted in the substrate and growing poorly, you might try floating the aquatic plant. It should do much better. You could float some of the aquatic plant, then occasionally take a healthy portion and plant it. When the planted portion begins to look poorly, replace it with another section of floating growth, which will be ready to prune by then.
The “tar” you see is probably a red algae (some red algae appear black). The best thing is to remove the bad leaves. Water sprite grows quickly enough to keep up with vigorous plant pruning. The earlier you remove spoiled leaves, the better. Also, if your planted aquarium is low on nutrients, water sprite will appear limp and lean, grow slowly and tend to accumulate algae on its surface.