Water Changes

[President Evil]

When we do our weekly water changes, ie.. 5% 10% 20% or whatever. Is the change percentage the total volume of the tank, or the actual water used? My 55 gal has lots of rocks and sand, so I'm sure that the water used is closer to 40 gallons.

[thegrun]

It is the amount of water you change divided by the true water volume of your tank and sump (tank/sump volume minus the displacement of rock, sand, corals and equipment).