Tropical Aquarium Plants
Since plants act as oxygenators only to an insignificant extent in an aquarium, it might be thought that they are useless or even deleterious. That such a view is completely erroneous will be apparent from the following remarks, outlining the principal functions of plants in aquaria. For convenience, these have been treated under six different headings:
1. To decorate the tank. This is put first even though it may seem the least utilitarian. If aquaria could not be made attractive or an asset to the appearance of a room, how many fish fanciers would there be? To be sure, there are many who are interested in fish per se, but by and large it is the beauty of a well-decorated tank that attracts people to the hobby. We believe that the vegetation of an aquarium is by far the greatest contributor to its beauty.
2. To minimize the likelihood of excessive multiplication of algae. In general, tanks that are well-planted suffer less frequently from green water or any other overabundance of algae. This phenomenon has also been noticed in outdoor ponds and pools and has been demonstrated experimentally out-of-doors. Several theories have been advanced to explain why it occurs. One is that the large plants shade the water sufficiently to prevent the growth of the small types, but it seems unlikely that this could happen in a glass-sided tank. Another idea is that the large plants produce some substance inhibiting the growth of the small ones, but no one has yet detected such a material.
A third theory has been suggested by Dr. C. M. Breder, Jr., and other workers. Since plant physiologists have shown that plants are dependent upon a number of substances for growth, a well-established stand of higher plants may use up one or more of the available growth-promoting materials as fast as they appear in an aquarium, or keep them at so low a level that no large amount of algae can be formed. Johnstone's application of Liebig's Law of the Minimum to planktonic growth is perhaps pertinent here: "A plant requires a certain number of foodstuffs if it is to continue and grow, and each of these food substances must be present in a certain proportion. If one of them is absent, the plant will die; if one is present in a minimal proportion, the growth will also be minimal. This will be the case no matter how abundant the other foodstuffs may be. Thus the growth of a plant is dependent upon the amount of that foodstuff which is presented to it in minimal quantity." Drs. F. W. Kavanagh and H. W. Rickett of the New York Botanical Garden have pointed out, however, that the amounts of nitrogenous and mineral substances used by plants in an ordinary aquarium are minute and unlikely to become a limiting factor. In fact, ordinary tap water is often a pretty good nutrient solution for plants, the very small amount of materials dissolved in it being sufficient for them. The reason a good growth of higher plants tends to prevent excessive multiplication of algae is thus a mystery, and if the Law of the Minimum applies, it does not concern the fertilizing substances in fish wastes since these are present far in excess of the plants' needs.
Many aquarists have believed that plants help maintain a more nearly uniform water chemistry in an aquarium by utilizing appreciable quantities of the nitrogenous wastes of fish. Because the amounts of these fertilizers necessary for aquatics' growth are insignificant compared to what the fish are producing, this effect does not operate to any extent. In this connection, it should be remembered that fish have been known to live in unplanted tanks for years without any change of water. They have also lived for long periods in standing aquaria kept in total darkness, where not even algae can grow.
3. To provide food for the fish, both directly and indirectly, by promoting the growth of various micro-organisms and other small animals. Fish that eat leafy plants are not popular with the great majority of aquarists, so the use of higher plants directly as fish food is relatively unimportant. Only algae are regularly eaten by the common species of tropical fish kept in aquaria. Indirectly, however, leafy plants and stoneworts may be quite important. The behavior of many fish gives ample demonstration of this, for they spend much time picking or grazing on the leaves and stems of submerged aquatics. In nature a great variety of very small creatures lives on aquatic plants. Undoubtedly fewer exist on the plants in aquaria, but, so far as we know, they help provide a more adequate and natural diet for captive fish. Nitella, one of the stoneworts or Charales, is especially valuable in this regard.
4. To shelter less dominant fish from attacks of their more aggressive neighbors. A careful study of the behavior of practically any group of fish in captivity reveals that certain individuals dominate others whenever there is competition for food, swimming space, or potential mates. Sometimes this domination appears to border on persecution, and the only way a less aggressive fish may be able to escape harassment from its more domineering tankmate is by hiding. Nothing can provide more or better hiding places in an aquarium than a generous growth of plants. It has also been demonstrated that dividing a tank into partially separated compartments reduces the amount of aggressive behavior among territory-holding fish, apparently by enabling them to establish their domains securely with less fighting. It would seem quite probable that plants act in a similar way, providing fish with niches, nooks, or crannies that they can call their own, so to speak, or as refuges when hard pressed by some other fish.
5. To provide sites for the attachment of eggs and refuges for the protection of young fish. These are well-recognized functions of aquatic plants. A large number of different fish place or scatter their eggs on plants. Among the better known genera that include species with such reproductive habits are: Hyphessobrycon, Hemigrammus, Nannostomus, Barbodes, Puntius, Capoeta, Carassius, Rasbora, Corydoras, Rivulus, Epiplatys, Melanotaenia, Pachypanchax, Aplocheilus, Pterophyllum, and Chanda. For baby fish, hiding places are life-or-death matters. Without a suitable sanctuary from hungry larger fish, few young would survive. A heavy growth of floating plants seems to be the best protection for tiny fish, and it also provides them with good feeding grounds for microscopic and near-microscopic prey.
6. To give the fish a natural habitat more conducive to their well-being—in the broadest sense of the term. This somewhat intangible function of plants is placed last because it forms a sort of catch-all for any items not included in the first five—most of which it overlaps in several respects. It is quite reasonable to suppose that, in general, captive fish will behave more normally, be more healthy, and will live longer and more natural lives the more closely their man-made environment approximates or betters their native environment. Plants form an integral part of the natural habitat of most tropical fish; for this reason, if no other, the aquarist should give them careful consideration.
Undoubtedly some of the functions of plants in aquaria have been omitted from the above. For example, Dr. Myron Gordon has found that Nitella can be used to condition raw tap water. It is nevertheless evident that fish benefit greatly from the presence of plants in their aquaria.
The benefits that accrue from keeping plants and fish together do not extend only to the fish, however; the plants, too, profit from the association. It is common experience that aquatic plants grow better when kept with fish than when kept alone, provided, of course, that the fish are not of the plant-molesting type. Aquatic plants need fertilizer just as terrestrial ones do, and fish wastes are evidently excellent fertilizer, although fish produce far in excess of what the plants require. Of much greater importance is the carbon dioxide given off by the fish. There is good evidence that this gas is usually a limiting factor in the growth of at least the higher aquatic plants. Without it photosynthesis cannot take place, and these plants apparently quickly exhaust the supply dissolved in water. Breder's measurements of carbon dioxide concentrations in aquaria lend support to this view, since he found that the carbon dioxide remained far below its equilibrium point with the atmosphere for long periods when the plants in an aquarium were actively photosynthesizing. A more adequate supply of carbon dioxide is undoubtedly the reason plants kept with fish grow better than those kept in old tanks rich in fish wastes
For both utilitarian and esthetic reasons, aquatic plants should be an essential part of the home aquarium, and successful aquarists will continue to use them generously.
In selecting aquatic plants, careful consideration should be given each individual species and its physical requirements. Following are discussions of the most important of these plants.
