Updated: May 12, 2019
The natural nutrient recycling carried out between plants and animals promotes a healthier captive environment for both. However, the co-culture of flora and fauna doesn't come without its own particular challenges. Ofttimes, the most frustrating aspect of culturing plants and fishes in the same space is herbivory: grazing of the plants/macroalgae by the fishes. While entirely natural (we generally welcome natural processes in our artificial systems, do we not?), hungry vegetarians can quickly decimate all plant life within the tight confines of an aquarium or small pond.
For this reason (and some others), plants may be sheltered and pampered in a safe (i.e. fish-free) space. In a pond, this might take the form of a bog filter. In an aquarium, such a space can be provided with a refugium. A keeper will in some cases lack the resources (space, cash, etc.) for the add-on. If he or she hopes to enjoy the ecological benefits of maintaining plants with fish--necessarily keeping them in a common space--then inedible or poorly edible plant types must be selected.
These herbivore-resistant species possess various types of defenses. But, generally, they either impede grazing activity (via thorns, tough fibers, etc.) or produce noxious substances.
Though it has rather soft and delicate foliage, ribbonweed (Stuckenia pectinata) is loaded with defensive chemicals. Like the legendary chemical warrior Eurasian watermilfoil (Myriophyllum spicatum), most of ribbonweed's chemical deterrents appear to be phenols and phenol derivatives.
A fascinating study by the Utah State University Ecology Center puts ribbonweed (and its extracts) to the test, pitting it against a most fearsome herbivore, the common carp (Cyprinus carpio). Findings suggest that while its phenols were not always distasteful enough to thwart grazing, they did appear to reduce the fishes' preference for the plant.