The majority of living plants require carbonic acid gas as a food, and, under the influence of light, decompose this gas, liberating the oxygen it contained. This is true of many of our common sea-weeds, and thus it is possible to establish in a salt-water aquarium such a balance of animal and vegetable life that the water is maintained in its normal condition, the carbonic acid gas being absorbed by the plants as fast as it is excreted by animals, and oxygen supplied by the plants as rapidly as it is consumed by the animals.
This condition, however, is more difficult to obtain in a salt-water aquarium than in one containing fresh-water life, partly because, generally speaking, the sea-weeds do not supply oxygen to the water as rapidly as do the plants of our ponds and streams, and partly because of the difficulties attending the successful growth of sea-weeds in artificial aquaria. Thus it is usually necessary to adopt some means of mechanically aërating the water; but, for the present, we shall consider the sea-weeds only, leaving the mechanical methods of aërating the water for a later portion of this chapter.
In the first place, let us advise the amateur to confine his attention to the smaller species of weeds that are commonly found in small and shallow rock pools, for the successful growth of the larger purple and olive weeds will probably be beyond his power, even though his tank be one of considerable capacity. The best plan is that we have already suggested—namely, to chip off small pieces of rock with tufts of weed attached, and to fix them amongst the rockery of the aquarium, being careful to place those that grew in shallow water with full exposure to the light, and those which occupied sheltered and shady places in the rock pool, respectively, in similar situations in the artificial pool.
For the purposes of aëration we have to rely principally on the bright green weeds, and preference should be given to any of these that exhibit, in their natural habitat, a multitude of minute air-bubbles on the surface of their fronds, for the bubbles consist principally of oxygen that is being liberated by the plant, and denote that the species in question are those that are most valuable for maintaining the desired condition of the water in an aquarium.
Any small sea-weed may be tried at first, but experience will soon show that some are much more easily kept alive than others. In this experimental stage, however, a constant watch should be maintained for the purpose of detecting signs of decay in the marine garden. A plant should always be removed as soon as it presents any change from the natural colour, or exhibits the smallest amount of slimy growths on the surface, for decomposing plants, as well as decaying animals, will soon convert an aquarium into a vessel of putrid and poisonous water.
It seems almost unnecessary to name a selection of sea-weeds for small aquaria, seeing that our rock pools produce so many extremely beautiful species, most of which may be successfully kept alive in a well-managed tank; but the common Sea Grass (Enteromorpha compressa), and the Sea Lettuce (Ulva latissima), also known locally as the Green Laver or Sloke, are particularly useful for the aëration of the water; while the Common Coralline (Corallina officinalis), the Dulse (Schizymenia edulis), the Peacock’s tail (Padina pavonia), the Irish or Carrageen Moss (Chondrus crispus), Callithamnion, Griffithsia setacea, Plocamium plumosium, Rhodymenia palmata, Rhodophyllis bifida, and Ceramium rubrum are all beautiful plants that ought to give no trouble to the aquarium-keeper.
It is not advisable to introduce animal life into the aquarium immediately it is filled, on account of the possibility of the water being contaminated by contact with the cement that has been used to make it water-tight. It is safer to allow the first water to stand for a few weeks, the weeds and all other objects being in situ, and the necessary means employed for perfect aëration during this interval, and then, immediately before the animals are placed in their new home, to syphon off the whole of the water, and refill with a fresh supply.
In the selection of animals due regard should be paid to two important points—first, the danger of overcrowding, and, secondly, the destructive habits of some of the more predaceous species.
No more than two or three animals should, as a rule, be reckoned for each gallon of water; and the proportion of animals should be even less than this when any of them are of considerable size.
As regards the destructive species, these are intended to include both those that are voracious vegetable feeders and also those whose habit it is to kill and prey on other creatures.