The odd fish is made from wooden barrels, which are connected by natural ropes. The lowest barrels should be loaded with sand so they could be like an anchor for the whole structure. Then the structure will stay in vertical position and it will be easier to detect it.
For this constructions it is needed to take 5 wooden barrels (400 l each) + 1 upper barrel of 500 l.
The common weight of such barrels is 125 kg.
The common volume is 2500 l (2,5 m3).
So it is needed to fill barrels with 1580 kg of sand. In this way the middle barrels should be with sand (1800 kg) + 290 kg of sand is needed to put in the side barrels (hands)