Fuel testing must be done daily, checking for both water and impurities.
It is important to check for the color change immediately after the fuel sample is mixed with the chemical in the tube. If you wait even a few minutes the sample will turn pink and you will have a false indication of water contamination.
Up to 5 gallons of fuel may need to be drained in order to obtain a clean sample
If more than 15 gallons of fuel are drained and the contaminants are still present, consider the fuel as contaminated.