Technically, 75% of 20 gallons is 15 gallons. But with substrate and décor, there will not be 20 actual gallons of water, more likely around 15 or less depending upon displacement. I assume the question is how to know you are changing 75%...I just drain out water until it is below the middle of the tank. I use a Python for the larger tanks so no idea of gallons or liters; with the 10g I use buckets and two (each holding 2.5+ gallons) does it, so with the displacement I am changing 5 gallons or a bit more which is certainly more than half of the actual tank volume.
