66 



YVES B. PLESSI8 



During the numerous tests which we 

 made we had never made a lateral opening 

 in the large pipe. We noticed, then, an 

 inconvenience of rather secondary im- 

 portance when this mode of procedure is 

 adopted: the inflow of the water which 

 most frequently is handled by means of an 

 exhausting device causes precipitations of 

 water that we later have tried to prevent. 

 Nevertheless, one of our installations has 

 made use of that very phenomenon to 

 create a "spindrift zone." 



The tanks with mud bottoms are con- 

 ceived along the same lines as those having 

 sand bottoms. It must be noted, however, 

 that the irrigation of the mud by means 

 of the water column of the large pipe is 

 extremely weak. Almost all of the water 

 passes through the overflow pipe of the 

 large tube. Too strong a pressure on the 

 mud must be avoided because the mud is 

 much less resistant to the "spring" phe- 

 nomenon than the sand. As to the surface, 

 it is particularly important to study the 

 water current so as to prevent its scooping 

 out the mud. Finally, when building a 

 tank of this type, it is advisable to pro- 

 vide, between the screen at the bottom of 

 the tank and the mud, a layer of a few 

 centimeters of sand. 



FILTERING SYSTEM 



The tanks with sand bottoms may be 

 located at any point within the closed cir- 

 cuit of an aquarium. They may be 

 mounted in series or parallel. It may be 

 well to note that they act as complete or 

 partial filters, and for that reason they 

 should be intercalated at suitable points 

 in the circuit. We have used these tanks 

 as filters in several different circuits. 

 They do not need any frequent mainte- 



nance operations and are able to go on for 

 years. Generally, they act as partial 

 filters, and the problem of complete filtra- 

 tion will have to be solved many times 

 without them. We have not used any per- 

 manent filters but have been content with 

 sand-bottom tanks acting as partial filters 

 since — in a complex breeding installation 

 where we have been more interested in 

 bringing about a biological equilibrium 

 than in the survival of a certain species — 

 such a filter traps the plankton and we 

 generally were eager to avoid such a trap. 

 But in a breeding installation where micro- 

 fauna and microflora are suspended in the 

 water and constitute a true plankton, it is 

 sometimes necessary to introduce a com- 

 plete filter into the circuit in order to 

 restore an equilibrium that has been upset 

 by an intervention of an experimental 

 nature. For that purpose, we have used a 

 large-dimension Pyrex glass tube, having a 

 diameter of 110 to 120 mm., and a length 

 of 80 cm. It was placed vertically into 

 the reserve tank where it exceeded the 

 water level by 30 to 40 cm. Its lower part 

 is closed by a plug of polyvinyl chloride 

 which is perforated and is kept in position 

 by plastic tubes that are reinforced inter- 

 nally by metal rods. These reinforced 

 tubes are held at the upper part of the 

 Pyrex tube by their ends which have the 

 form of hooks. We introduced empty 

 shells into the filter, at its bottom, in a 

 layer having a thickness of 5 cm., followed 

 by a nylon screen which holds a 20-cm. 

 column of charcoal ; this is followed by a 

 thin layer of glass wool covered by small 

 pebbles in a thickness of 15 to 20 cm. This 

 filter is kept in place during the time re- 

 quired and is maintained at a suitable 

 level. 



MODIFICATION OF THE SAND MILIEU FOR A PUBLIC TANK 



The installation with a sand bottom that 

 has been described may be of great interest 

 in a public aquarium for the presentation 



of psammophile anemones, pleuronectids, 

 sand crabs, etc. In large sizes, this ap- 

 paratus can be installed with two lateral 



