U do the math.. or help me do it.

[Dissident]

Tank 110gal, heavily planted.
5" Flourite Substrate and an LARGE piece of driftwood. So for sake of argument lets put total volume of water at 90gal.

Due to the custom filters I can only change about 1/3 of it at a time ~30gal and refill. Lets say I do another waterchange of 30gal in theory only 2/3 of that 30gal would be old water (~20gal old water).
Would that be about right then 2X30% WC on 90gal of water would remove 50gal of old water?

If i was to do int a 3rd time would that mean I am not only changing 1/3 of the old water? The logic of that does not seem to ring right to me. I know I am missing some unknown value here any help would be great from you math gurus out there.

[goheel]

Before you change the 3rd time, you would have 50 gal of new water and 40 gal of old water. So if you change the third time, I guess you would get about 13.333 gallon of old water. You also have to take into account that the old water will diluted into the new water, so I don't think there's a distinction between old and new after once they're all in the tank.

[Alight]

You're close. After two changes you'd be changing 50% of the old water. After the next change you would have changed 63.3% of the water.

[Dave C]

You're close. After two changes you'd be changing 50% of the old water. After the next change you would have changed 63.3% of the water.

You're close. After two change you'd have changed 50 gallons of old water, which represents 55.6%. After the next change you would have changed 63.3 gallons of old water, which represents 70.4%.

[Dissident]

LOL this is great thanks. I knew it was going to be rocket-science but once we get together we can find a cure for world hunger.:D

[Alight]

Oops, sorry. You're right Dave. I did my calculations in gallons, not percent!! Forgot to convert back.

BTW, I'm not a Rocket Surgeon, but I am a Brain Scientist! (LOL)