ome disadvantages, notably those of expense, weight and
brittleness, but are often used. Better, cheaper and lighter, if less
permanent, are tanks of wood bolted together, pitched internally. Glass
bell-jars, useful in particular cases, should generally have their sides
darkened, except when required for observation. Provision should always
be made for cleaning every part of the tanks, pipes and reservoirs; all
rock-work in tanks should therefore be removable. As regards the
lighting of fixed tanks, it should always be directly from above. In all
tanks with glass sides, whether large or small, as much light as
possible should be kept from entering through the glass; otherwise, with
a side-light, many animals become restless, and wear themselves out
against the glass, affected by even so little light as comes through an
opposite tank.

In cases where distance from the sea or other causes make it
impracticable to allow the overflow from the tanks to run to waste,
special precautions must be taken to keep the water pure. Chemically
speaking, the chief character of the water in an aquarium circulation,
when compared with that of the open sea, lies in the excessive quantity
of nitrogen present in various forms, and the reduced alkalinity; these
two being probably connected. The excess of nitrogen is referable to
dead animals, to waste food and to the excreta of the living organisms.
The first two of these sources of contamination may be reduced by care
and cleanliness, and by the maintenance of a flow of water sufficient to
prevent the excessive accumulation of sediment in the tanks. The
following experiment shows the rapid rise of nitrogen if unchecked. A
tank with a considerable fauna was isolated from the general circulation
and aerated by four air-jets, except during hours 124-166 of the
experiment; column I. shows per 100,000 the nitrogen estimated as
ammonia, column II. the total inorganic nitrogen:--

I. II.
Sea-water at source of original
supply 0.001 0.003
Aquarium water in tank at
commencement of experiment 0.012 0.400
After 22-1/2 hours 0.020 ..
" 75 " 0.025 1.200
" 93 " 0.019 ..
" 121-1/2 " 0.012 ..
" 141 " 0.015 2.200
" 165 "