Yes. 20%.
It seems to me that what most bothers the fish is a sudden change in their conditions. Particularly the water chemistry, but my personal observation is that temperature variations seems to matter less than chemical changes.
So, on one hand if you do a water change every week, fortnight or month, then the incoming water is going to be significantly different chemically to the water you're taking out. Bigger changes logically will cause more of a shock.
But, if you can do what I do, then a 5% change 4 times per day (total 20%) means that there is little to no chance of any major chemical buildup relative to tap water. That 5% change happens by plant dripper over a 30 minute period, so any temerature changes are going to be lost in the noise.
Having this much water change means that the fish seem to do well even in what would be normally a case of serious overstocking. Pet stores get away with high stocking levels by having massive filtration capacity and daily water changes from the sump as part of their routine. Likewise, I seem to be able to get away with having an unthinkable number of guppies in a 20 gallon tank at times. Not that I like doing it, but sometimes they just breed too fast and it takes a while to reach equilibrium.
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