50 THE FRESH WATER AQUARIUM. 
able cleaners, the pity is, that they will not see _ is 
required of them, and do that only. 
Again, the univalve mollusks do not keep the sides so 
clean, but that an occasional cleansing of another kind is 
sometimes necessary, and if the aquarian is not disposed 
to keep an army of quite semi-efficient scavengers, the 
remedy will be found in an occasional cleansing of the 
sides, by means of a sponge attached to a stick, which 
must be plied over the whole surface, and occasionally 
taken out and washed in clean water, tc remove the green 
scum, that it soon gets covered with. 
Use of Confervoid Growths——But I should object to 
any frenzy about cleansing tanks. As I said at starting, 
they should be self-supporting, and if Planorbis or Palu- 
dine are used in the proportion of about four of each to 
every gallon of water, a good view will always be pre- 
served with the use, now and then, of the sponge alone. 
- Periodical Cleansing —When a tank has stood twelve- 
months or more, the water not having been changed at all 
during the whole time, it may be necessary to turn out the 
contents and re-stock it. Thisis not to be done unless the 
bottom has become black, and the roots of the plants show 
signs of decay, in fact, not unless it really wants it, and if 
bottomed with mould it certainly will, and it must be 
done accordingly. The live stock should be removed by 
means of a hand-net, the water drawn off by means of a 
syphon of glass, or gutta percha, and the plants taken out 
carefully and put by themselves, and then after removing 
