Originally Posted by Maridia
Do all 20. You want to take out all the bad water and replace it with nothing but good water.
Second that, do it all at once. If you do small water changes back to back you actually end up with more dirty water left in the tank which defeats the purpose. Generally you want to do smaller water changes more frequently but if your water is really dirty then one larger one is better. Let's look more closely shall we, if your current water has levels around 1 ppm
of nitrate and you do a 50% change it takes it down to .5ppm. If you do a 5gal change that's 8% and will drop your nitrate to .92 ppm
. If you do this 4 times for a total of 50% or 20gal you will end up with a nitrate level of .716 ppm
. So doing 4 seperate 5gallon changes basically cuts the effectiveness of a 20 gallon change almost in half compared to doing one 20 gallon change. From my experience stability is important in an aquarium and more specifically in reefs however removing dirty water and adding clean water in larger volume usually doesn't cause ill effects quite the opposite. Most of the time your tank inhabitants be it coral or fish will respond very positively to the large amount of clean water..