72 NATURE'S REALM. 
sufficient quantity. Reduce the number to about 
thirty, introduce two or three more species of 
aquatic plants, also some Tuffstone rock work, 
and you will find that your tank looks as good, 
even better, than betore, and the remaining fish 
will thrive better. 
That your fish rise to the surface and are in 
distress in the evening, while they are at the 
bottom and apparently comfortable in the 
morning, seems to prove that they have not the 
proper amount of oxygen. The burning gas 
in your office consumes most of the oxygen 
contained in the atmosphere, and the water 
can therefore absorb but little. 
The best way will be to empty your tank. 
Clean the bottom and the (inside) glass sides 
with common table salt, using your wet hand 
instead of a sponge or rag. Wash the sand 
and pebbles in several waters, then replace. 
Plant the plants, place the rock work and ar- 
range a couple of handfuls of pretty good sized 
pebbles here and there over the sand, taking 
care, however, to place the individual pebbles 
not too close to each other. There are to be 
— 
little spaces, cavities, on the bottom of an aqua~ 
rium wherein the excrements of the fish and 
other loose. particles can collect, so the fish 
while swimming near the bottom cannot mix 
these again and again with the water. This is 
the mission pebbles have to fulfil in an aqua- 
rium. 
Everything replaced carefully, refill your 
tank, making the’water the same temperature 
the fish are accustomed to, add a teaspoonful 
of table salt and reintroduce the fish, together 
with about a dozen of frog tadpoles, which act 
as scavengers. 
Once in a while the sediment that collects 
between the pebbles should be removed by aid 
of asiphon. Let your aquarium have no sun- 
light. 
The plant of which you sent a sprig in your 
letter is the common wax or hornwort (Cerafo- 
phyllum demersum, from keras—the horn 
or wax, and phyllum, the leaf, owing to the 
waxy or horny texture of the plant). It is a 
good oxygenator, although one of our com- 
monest native aquatic plants. 


