TROPICAL AQUARIUM FISHES 223 
drying. For outdoor pools an inside slope of 2 inches is about right. 
This would be, for a 7x 9-foot pool, a top thickness of 4 inches, with 
6 inches at the bottom. Base, 6 inches thick. For larger sizes add 1 inch 
of thickness to walls and base. Indoor pools require no slope. In mak- 
ing all calculations, allow 3% inch for thickness of finishing coat. The 
forms should be levelled up and made absolutely true in the beginning. 
It is a bad plan to depend upon the finishing coat to correct inaccuracies. 
It seldom does it. Some considerable care is necessary to establish the 
four corners at perfect level. Select one corner and measure each of 
the other three from that. If the spirit-level is attached to a board, 
its trueness should be tested by reversing the ends. In any case the 
level should be reversed on each test. 
The concrete is mixed 1 part cement, 2 parts sand, 4 parts 34-inch 
crushed stone or round stone. This is poured to within Y inch of top 
of form, beginning with the base and must be well tamped to avoid 
bubbles and open spots. Those wishing to secure the best possible 
results first place a thin skin of cement over the cinders so the water in 
concrete will not seep into the cinder base, it being desirable that all 
concrete work dry slowly in order to crystallize perfectly. This prelim- 
inary skin is as thin as possible and is applied the day before. It is com- 
posed half each of sand and cement. If this is used, the lower layer 
of re-inforcement may be laid directly upon it. 
Fic. 166 
Fig. 164. Tile Seton Cement Mound. Fig. 165. Top Finishing Coat Brought to Level of Form, Fig. 166. Guide-board 
in Position for Inside Finishing Coat (Whether or not tiles are used, this is the best method of finishing) 
Finishing coat should be applied within 24 hours, and is made 1 part 
cement, 2 parts sand. Finish one side at a time, top edge first, inside 
surface next, then outside, and the bottom after 4 sides are complete. 
Begin by filling to top of form. Now take off inside form from one 
side only. Lay a piece of perfectly straight board along top edge. This 
makes an infallible gauge for thickness and straightness of finishing coat 
(Figure 166). Here we might say that it is advisable to have an experi- 
enced finisher do this work, but a resourceful amateur can do it. Now 
do the outside in the same manner, the idea here being to have the top 
bind with the sides while fresh. The outside being done last is less likely 
to be kicked. Try to protect the job from rapid drying in the sun. 
