If the second sample is contaminated, the tank should be cleaned at the first convenient opportunity. Filters and sediment bowls on overhead tanks will minimize the chances of contaminating equipment fuel systems. The filter should have enough capacity to pass fuel at the usual rate of filling. Most fuel pumps are equipped with filter screens which limit sediment contamination, but require regular cleaning. Most plastic tanks aren't suitable for above ground fuel storage.
Sunlight going through translucent tank walls promotes microbial growth and other fuel degradation.
In addition, most plastic tanks are not strong enough for safe fuel storage. Evaporation Evaporative losses vary with the type of storage, type of fuel and steps taken to prevent evaporation. Losses range from minimal in underground tanks, to easily measurable in poorly located and maintained overhead tanks. In one test, as much as 30 litres per month was lost from a 1,200 litre tank. The greater the swings in temperature of the fuel, the greater the potential loss and the larger the risk of contamination due to condensation.