100% water change

[jdr3366]

Hi,

If I have a 180 gallon tank and I replace 60 gallons of water each day. How long will it take for me to have changed 100% of the water that was in the tank on day one?

Is there a simple formula?

Thanks,
John R

[Ralph]

It'll never happen, mathmatically anyways. It's like cutting something in half, then cutting the half in half, etc. Eventually it is so small that it can't be measured, but theoretically "never."
But I assume though that you are asking about the level of waste products in the water. Doing a 30% change every day (which is pretty good, especially on a big tank like that), you end up with between one and two day's worth of waste in the tank (depending on when you measure it in the day).
What you are doing with the water changes is diluting toxins in the tank and you will always have them present, but at levels that your fish can handle.