2 the material can be thioseal or silicone filler as diagrammatically illustrated in fig1 an overflow is provided at the periphery at the top of the pool into which water will flow and a water inlet 50 is connected to a pump 51 which the water through a filter 52 to a header 53 which is connected via respective inlets 54 with control valves 54 39 to individual channels of the tube tile in this way continuous circulation of water is established in the pool after water has been pumped through the inlets 54 into the channels 20 21 22 in the tile tubes the water will traverse the channels and flow outwardly through the holes 30 33 into the pool the alternating arrangement of the lines of holes 31 and 32 of each tile tube improves the mixing of the water into the pool while the inclination of the holes 30 and 33 at the ends of the tile tube tends to direct the water towards the laterally adjacent tile tubes fig9 and 10a 10c show a control element 80 by which the degree of opening of the holes in the tile tubes can be regulated the element 80 is tuneably mounted in the tile tube and includes a valve 81 mounted in a recess 82 in the flange in the tile tube at a location where two holes 32 are adjacent one another the valve 81 has parallel sides 83 and as seen in fig1 a the angular position of the element can be such that the outlets of holes 32 are blocked in fig1 b the element 80 is turned 90 u00b0 and the outlets 32 are completely open.