In Bome cases, where certain dangers occur regularly, the practical fishbreeder should 

 be able to exercise a certain and sinple control over the chemical conditicHis of the water. 

 It is for this reai^on that we set forth here some of the principal chemical properties of 

 the water, giving at the same time some information for their proper detection. 



Oxygen content . 



Oxygen is not only l^iportant for the existence of Ush — ^their respiration — but also 

 for the existence of all living organisma in the pond, with the exception of seme species 

 of bacteria. 



Cnly water-dissolved oxygen is utilizable for respiration (i.e., the form in which 

 it escapes by the boiling of water), but not the combined cocygen. 



Now, in a pond, we deal with oxygen consumers — especially certain bacteria — as well 

 as with CDcygen producers, such as the green water plants. 



The oxygen content of the water always tends to maiiitain a normal value toward the 

 saturation point, and viiich varies according to variations in tenperatures , 



TSe have here the following figures for this "normal rate"j 



at centigrade lA.57 mg per liter, 



at 5 " 12.74 mg " " 



at 10 « 11.25 mg " " 



at 15 " 10.07 mg " •« 



at 20 " 9.10 mg " " 



at 25 » 8.27 mg » " 



at 30 " 7.52 mg " " 



We see fixjm these figures that a rise in temperatures causes a drop in the satxiration 

 points, TJhen the saturation point falls below nonaal or rises above it, it leads to an 

 interchange with the surrounding air, and at a more rapid rate, the greater the deviation, 

 i.e. from normal, and the greater the surface of interchange in proportion to the volume 

 of water. 



This — in conjunctions with previous discussions— explains offhand why lack of oxygen 

 is especially found ; 



(1) In the water of springs and in piped water (usually shut off from direct 

 contact with the air for long periods of time) and where no plant life 

 supplies new oxygen to balance the continuous cOTisumption of it. 



(2) In winter ponds without a current, shut off froa air and light — through 

 snow and ice — for quite some time, 



(3) During the early morning hours of hot summer days. At such times, the 

 saturation point is low and the "exhaustion" (i.e. the loss in oxygen 

 during 2A hours and at a ten^jerature of about 20 centigrades by complete 

 exclusion of air and light), and the consua^jtion of oxygen on the part of 

 fishes and aquatics in high; in addition, the plants have not produced any 

 oxygen during the hours of the night (due to lack of light) but have con- 

 sumed oxygen. 



(4) TShen putrefactive substances are present in the pond (waste waters from 

 sugar i^fineries, from starch and cellulose factories, from breweries, etc. 

 Sewage waters from cities, mowed reeds, hay, decaying algae and submerged 

 plants in late summer), and especially in summer time and also shortly 

 after the freezing over of the pond. 



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