As far as one over the other's effect on dilution, 4x20% water changes remove 10% more solids for only 4X the work. I don't know many people other than discus breeders who change their water nearly every day, but if that sort of thing is what floats your boat, then sail on.
Of course the point becomes completely moot if your once a week, large water change is more than 59%.
So I submit that changing 61% of your water weekly is superior.
Now what you should do is make a post arguing that 7x20% water changes in a week is a better method, to which I will counter that changing 75% of your water at a time, once per week is better. Maybe we could even go so far as to argue that changing 20% twice a day is the real way to go.
Can you see where I am going with this?
How dare you suggest my % WC's are not precise measures of the data, you found me out.
It sort of all comes back to what is really the risk of the build up and toxicity to a variety of plants, algae, livestock, shrimp etc. No one has done that, so the debate is a bit silly without some end point goal.
Automated systems I've installed for 3 client's, and the float switch does the rest, one TPT member dialed his phone to change his water at home while at the AGA convention..........
All the techys were all "OOOO....... ahhh"
Water changes are a useful tool for managing nutrients, I'm not sure there's any real evidence that more or less really helps, but it does make the ranges more known and stable.
I'm not sure stable is really good or bad, I've not seem much evidence there either.
But..........it does not take long to do 61% or 80%, vs 50%.
I just chose 50% to simply the max build up from dosing for the week, and many discus folks typically already do 50% 1-2x a week. Nothing can build up more than 2x the dosing for the week. That's simple and easy to understand.
At the time, I was breeding them and noticed the tank did really really well.
If you run the dosing EI modeling calculator, you will not that more frequent smaller changes keep the tank more stable ppm's wise..........but the question is ..is that better and if so, how? What evidence is required to demonstrate that? I've looked for oh, 15 or so years and have not come up with any decent reason I could verify.
And could you do 25% instead of 10% or 20%?
Sure, but if you wanna drag out the hose and do the water change.........it simply does not take that much more labor or time to change 20% vs 50%.
Like I said a long time ago, if you bother doing them, may as well go big.
No worming around the common sense and the human motivational part there.
for those wanting to invoke the "Well it works for me" or "I'm just sharing and different method"...here's my response to that and it requires Zero water changes
, no $$$ cO2, and no labor to speak of, feed the fish(So I do technically dose) and that's pretty much it, bred dozens of fish in here too:
Show me your tank that has no water changes for 6 months?
I mean if "less is better" and all..........
Go whole hog, stick with a philosophy, and run with it........ this ain't a buffet, you do not get to pick and chose to suit.