Since 450 liters represents 45%, then each 1% is equal to 450 liters/45 = <<450/45=10>>10 liters.
Hence, the capacity of each tank is 10 liters x 100 = <<10*100=1000>>1000 liters.
So, the first tank needs 1000 - 300 = <<1000-300=700>>700 liters of water more.
While the second tank needs 1000 - 450 = <<1000-450=550>>550 liters more.
Therefore, 700 + 550 = <<700+550=1250>>1250 liters of water are needed to fill the two tanks.
#### 1250