We need to find the total change in the amount of water in the tank after 6 days.
Since the water tank decreases by 10 gallons every 3 hours, this means it decreases by 10/3 = <<10/3=3.33>>3.33 gallons per hour.
In one day, there are 24 hours, so the water tank would decrease by 3.33 * 24 = <<3.33*24=79.92>>79.92 gallons.
Therefore, in 6 days, the total change in the amount of water in the tank is 79.92 * 6 = <<79.92*6=479.52>>479.52 gallons. Answer: \boxed{479.52}.
A water tank decreases by 10 gallons every 3 hours every day what is the total change in the amount of water in the tank after 6 days?
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