*• ■ ■ I have tried to determne hov; often conplete changes of vmter 

 in the tr.nlcs took plnce» To that end I dissolved in one tanlc with 

 a "voluirie of le4 cubic meters one grr.m of potassium permanganate. 

 VlD-th the wooden slides over the windows this was first thorou^jhly 

 stirred into the vmter after which the wooden slides vrere removed. 

 Beforeliand a nvunher of solutions had been prepared ranijing from 

 1 : 1000,000' to 1 : 20>000,000 and placed in glass tubes to equal 

 heii^hts. 



I'Tith these we compared the samples periodically taken from the 

 tank. At 8.02 p.m. when the slides- 'vj-ere removed the first sample 

 was taken and this and subseqtient samples ran as follovTs: 



time: Concentration 1 in: 



8:02 •' between 1 and 2 million, closer 



to one million which agrees with 

 the confuted concentrrtion of 

 1 : lo4 million 



8:16 4,000,000 



etc. 



"8:40 color still visible 



, 8:50 " " 



9:00 color entirely invisible 



After 35 or 40 minutes the refreshment of the vrater vras so 

 strong that the solution had been v<reakened more than 20 times and 

 one mr.y safely say that after one hour the water wnxs completely 

 cliangede 



Attempts \7ere also i.iade to determine vmether the oxygen con- 

 tent of the tardcs was also sufficient. Especially in still, vmrn 

 nights, when plants also absorb much more oxygen than they expel, 

 it seemed not im.probablo that there v.-ould he a shortage. To 

 ascertain this I used the titration method after "ffinlclo-r, accord- 

 in:: to the nodification, given by the Codex Alimentarius for 

 testing vrater. This modification seems hov/ever to be unrelia'-lo 

 ■ for testing sea water which agrees with my ovm observations. Thus 

 it would have no value to publish the figures obtained although I 

 coul'''- determine,' from -the typical and similar anrlyses made, that 

 there can Le no question of a lack of oxygen even at night and 

 that the pr -.port ion vrithin and oTatside of the rearing tanlis was 

 practically t?ic same, ' 



In 1919 much tmuble was caused by crustaceans which in an 

 undetermined rcnner but proba'jly as- eg s or young: larvae had 

 entered tho tanlcs. Thus vfo found shrimps, varieties of S_ch_i zopods 

 as for 'example Praunus sp.., A mphipods (Garp'::ar us ,_sp . ) young speci- 

 mens of ordinary s'hore-'crabs etc^ Also young fish (herrin:_:, harder, 

 little grondel) 'wore regularly to be found* 



141 



