A black, slimy layer is growing on my plants – what is it?
You most likely have a blue-green algae infestation. This can be an annoying problem in aquariums and when covered in large amounts of algae, your plants’ leaves are no longer able to photosynthesise. Furthermore, blue-green algae can release toxins into the water, which can make your fish ill. Blue-green algae particularly flourish in water with too high a nutrient content or if the substrate is poorly maintained, for example too full of mulm. Proper water and substrate care should generally prevent the growth of blue-green algae. In the case of an acute infestation, it can help to leave the aquarium in complete darkness for several days. To combat the algae, try partial water changes at a higher volume and, where possible, more extensive substrate cleaning in addition to your scheduled care measures.