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.