TROPICAL AQUARIUM FISHES 11 
aquarium. As the air passes through the water there is sufficient oxygen 
absorbed to keep the fish always in good condition. This is also very 
helpful to the fish at night. 
There are many varieties of pumps that can be used for this purpose, 
but the cheapest and most satisfactory are those which are operated by 
water power. The type known as “beer pumps” are the best. These oper- 
ate on a very simple principle, having a minimum of working parts and 
therefore little to get out or order. Air is forced out of an air-chamber 
by the entrance of water. When the air is all expelled, an internal float 
stops the supply of water and starts a siphon working which empties the 
pump preparatory to the next filling. As this operation takes about a 
minute it is advisable to have a small storage tank for the air to pass 
through in order to equalize the flow. A very small stream of air run- 
ning through the aquarium will keep the water sweet even though the 
aquarium is somewhat overcrowded. Overcrowding, however, is not to 
be encouraged at any time. 
The air should be liberated in as small bubbles as possible. Libera- 
tors are made especially for this purpose, but a good home-made plan is to 
place a piece of bass wood or other open grained wood in the end of a 
tube to force the air through it. In case the liberator becomes clogged up, 
allow it to dry out for a day or two. 
An air pump may also be used to operate a filter for the aquarium 
or to make a fountain without the use of water other than that already 
in the aquarium. (See Chapter on Aquarium Appliances.) 
Scavengers. Nature has supplied us with means of getting rid of 
most of the harmful offal and decomposition in the aquarium. These con- 
Toa) 
30 
Fic. 2. African Fic. 3. Japanese Fic. 4. Red Ramshorn 
Tue THREE Best FRESHWATER AQUARIUM SNAILS (Life size) 
sist largely of those species of snails that do not attack the plants. Among 
the best known, most satisfactory and easily obtained are the large Japan- 
ese snails (Viviparous malleatus), the so-called African paper shelled 
